My favorite lighting is MH with blue led powerbrite modules for exteneded viewing of the tank. Every system I have owned has been with a canopy. I have never had heat problems but we do keep our thermostat at 74...and I know if you keep it any higher you could very easily run into heat problems. I also only run my MH as long as I can keep the tank under 81. Over the years depending on the system and size of tank I have been able to run MH in a canopy from 6-8 hrs per day.

I currently have a 55g with canopy and T5 lighting along with led powerbrite modules for the shimmer effect and love it.

My 85g will be lit by two 250w MH in a canopy. If I have any issues with heat I will put a fan over the sump water that turns on at the same time as the lighting.

So...it can be done but you just need to pay attention to a few things as I have mentioned.