I'd like to put on a public "maker fair" demonstration of green screen filmmaking. After Effects and I are old friends, but I'm hoping to find or hack together a live review system that won't put me into debt.

Has anyone else looked into this, or have ideas?

Wirecast is the only software-only product I know of that does this. Blackmagic's ATEM switchers and the Tricaster switchers do it, too. But they cost lost of money since they are both hardware and software.

Good suggestion, Wirecast, thanks!