I'm the author, if anyone has questions feel free to ask. 0.6 should be released this month with a lot of improvements.

How large does the heap need to be for the garbage collector to work effectively? Could this theoretically run on something like the TI Stellaris LaunchPad, which is an ARM chip with 256KB of Flash and 32KB of RAM?

The default heap is 2MB, but will run with 756KB. If you want to skip the init.scm file (-q command-line option) it will run with just 10KB providing only definitions and C primitives, but I haven't tested this extensively - I had no idea they still made devices with such little memory :)