I can see this weekend if a 2800 will fit, I have one that's not installed in anything right now. I believe it's the same size radio. Yeah heat is a concern, but others seem to be doing it with no ill effects, so
your call on if it's an issue. Depends on how much you transmit, and at what power, and how well tuned your antenna is... kinda hard to get a straight answer. IMO the remote faceplate under the heater is best, with the head unit either under the passenger seat or behind the back cargo panel.
And yes, you could technically blast any frequency through any antenna with some reduced capacity, sure -think of all the home brew antennas out there. But the performance loss on transmitting gets converted to heat, which in a worst case would potentially let the smoke out of the radio? I'm sure a few transmits you'd be fine, but prolonged use probably not a good idea. It's easy enough to get a dual band antenna in the first place though, I think the SBB-1 goes both ways and it's a cool little stubby for trails.