I dumped some nightcrawlers in my compost and contrary to what I'd heard, they did not make off in search for better compost - apparently they like our diet and are sticking around to work on the refuse

I think that providing an appropriate temp gradient in a small space is hard enough
even if you get a small process going which might be hard, but perhaps not impossible, it's gonna be a long time to get it to stable heat

I think it's a very interesting long-term project
I guess you'd need some insulating medium to make up for piles of plant matter

btw
there is an article in the current "Reason" magazine by Brian Doherty, titled "Power from the People", that might interest you
I was looking, but it's not on their web-site yet
it's about making machines that essentially run on certain kinds on garbage and are carbon neutral