Any sort of under-tank heating may help but it really depends on the species. If the species requires high humidity a water bowl could be set over the area where the heater was and the increased evaporation would help heat the air. But then you'd have to reduce the ventilation of the cage front (it's former top). This will work well for species that will thermo-regulate to a hide box on the floor.
If you only need a slight heated area heat pads/tape can be taped to the back or side of the cage. Arboreal hides will increase the temp of the localized area. Again, this idea will work better for certain species. Geckos that perch on the glass, for example.
Clamp-on light fixtures can be positioned to shine light/heat into the cage through the screened portion of the lid.
Lastly, radiant heat panels might work without cracking the glass. I'd e-mail Pro Products and ask.
The last two methods will be the most versatile for a variety of arboreal species.