That's pretty cool! I still have an original MacBook1,1 (so i386 and came with Mac OS X 10.4 from the factory), I think it should be able to run PPC binaries using Rosetta.
For scaling up to 2x, I'm sure you can get the scaling mostly "for free" in terms of CPU usage by using OpenGL for the scaling. Not sure how easy that would be to integrate with Allegro, though.