While we're right to be wary of light when it comes to finished beer, worries about heat are - persistently and irrationally - overstated. While heat does have an effect, it isn't an inherently damaging factor in its own right: it needs help. And, by and large, if you're producing good, healthy beer then you don't need to worry quite as much.
Bright vs. Hot
Light is our enemy. We've all tasted skunky beer. Skunking is an effect caused by the interaction of UV light with specific compounds found in hops (though not hop extracts, as I understand it). This has led some to conflate light with heat, which is a bad idea for at least two reasons.
First, it might make some less wary of light exposure in cold environments. Be wary. Chilled beer that's sitting under fluorescent lights, or out in the sun on a cold day, is probably going to skunk quickly.
And second, there's very little reason to be concerned about heat, in and of itself. Some have written about the potential for the formation of coagulated proteins due to extended hot aging (which I'm not scientist enough to support or rebuke), but I can't recall a specific time when I experienced it. Reading others' accounts the effects seem to be limited to mouthfeel and head formation which, while important, aren't at quite the same level as light effects, which can make your beer taste like homeless-person asparagus urine.
A Natural Experiment
One reason I'm pretty confident that heat isn't a principle concern in terms of beer flavor and its stability is because I'm a bit of a nerd. I generate and keep data on my beer, and one of the things I track is competition score as a function of age in the bottle. It gives me a sense of overall flavor stability, among other things, and one year I was provided with a nice little natural experiment.
I moved. And in the course of moving house, my finished, to-be-sent-to-competition beer had to be stored for 32 days in a garage. In the middle of summer. During several heat waves.
Now, ordinarily my "evaluation" beer doesn't see the light of day between bottling and submission, and it's stored in a near-freezing refrigerator. So as you can see, this was beer abuse - if we start from an a priori position that heat is a detriment to beer.
Well, I now had my natural experiment. 18 beers that had been hot-aged for a month, vs. those in previous (and subsequent, eventually) years that had not. I compared the scores for those beers to those of other beers of similar style and age, and also compared the average effective "life" of each (the point at which its flavor and/or scores fall off the cliff, using judges' scores/comments as well as my own organoleptic evaluation).
I'll spare you the statistics (though I hope to present them in some detail at a future Homebrewcon), but suffice it to say that there was no statistically significant decrease in scores or life expectancy for the treatment ("hot-aged") beers.
Scoring fluctuation was within "normal" limits (there's always a bit of variability in those due to the human nature of beer judging).
Flavor stability was unaffected (and, in fact, for the lagers I got a very counterintuitive result when their timeline actually extended). I can count on more than a year (13.1 months, to be exact, as a 20-batch moving average) of flavor/scoring stability, and this batch had no issues there. The hot-aged beers varied in initial bottling date, too, so some were hit with this within a few weeks of bottling, and others after more than a year. As a group, and individually, there was no statistically (or substantively) significant change.
And for what it's worth, they tasted fine, too. But that's so highly subjective that I shudder to even mention it.
When Heat Matters
Maybe I just got lucky. It's certainly possible. But I don't think that's it.
Heat has one undeniable effect on beer (and most any other chemically-reactive situation): it speeds up reactions. Arrhenius tells us that for every 10C increase in temperature, reaction times double. Now, many have taken this rule and oversimplified it to state that "heat means beer stales faster." That's incomplete.
Heat certainly might make beer stale faster, or sour faster, or anything faster - but only if the requisite process(es) is (are) already underway. A beer with only limited contamination might be "turning" faster than it ordinarily would have in a cold environment, but that doesn't automatically mean that it will hit detectable levels before its normal timeline runs out.
So I'm not denying Arrhenius' rule, I'm just saying that it might have only limited (insignificant and/or insubstantial) effects depending on the underlying beer's situation. If you have some history of producing contaminated beer, then, you should certainly be avoiding heat.
Heat is also blamed for accelerating oxidation, but that, too, is incomplete. Subject to Arrhenius' rule above, oxidation (if already present) may be brought to the fore more quickly in hot-aged beer. What many are blaming on heat, though, might really be a function of temperature fluctuation, not temperature level. As temperatures change, your beer bottles (and their caps) are expanding and contracting. That temperature change is also causing pressure changes, and these two elements are very likely resulting in the introduction of more oxygen into your beer. Hence, greater risk of oxidation.
So yes, temperature swings might be present in a hot-aged environment, but apparently they weren't a huge issue in my dad's garage. And if you gave me the choice between aging at a steady 85F vs. aging in a cooler room with 30F temperature swings, I'd take "hot" every day of the week.
Respect Heat, but Don't Fear It
In a perfect world, as soon as your beer is carbonated to your target level, you should be storing it cold. But if circumstances make that a challenge, you shouldn't abandon all hope. Make your priority temperature stability, and keep your fingers crossed that whatever's happening faster in that bottle isn't going to catch up to you!
And if you notice that it is...well, I guess it's time to get rid of it. Cheers!
Keep it simple.