In short, like it or lump it.
Ah yes - the voice of experience.
Bollocks. I've refused hot planes at two of my three 121 airlines. One of them was a CARGO PLANE. And I'm still employed.
I live and work part-time on a farm in South Alabama, and know more than a little about heat-stroke (the clock says 95 F right now). I was preflighting one of Fred's up in MEM in August last year, 12 noon, temp about 96, zero APU, zero ground air carts. I stripped to my waist and began my prep. After 25 minutes, and despite consuming two quarts of water, I stopped perspiring and got real light-headed. Called GOC and told them I was done. Next time I won't take my shirt off - why delay the inevitable.
The other was a pax EMB-120. I didn't even go out to it (ATL in the summer). They found a non-union instructor ('bout like you 411) who took it anyway. The copilot told me they took off even their T-shirts in the cockpit. Guess their pax all survived.
Some of these posts make me sick. Imagine if someone's grandma had died from heatstroke. That captain would be in jail today. GROW A PAIR. GATE AGENTS CAN'T GET VIOLATED.