hmm... I think you could maybe figure something out, but it will be a challenge. I think volume might be your biggest issue. We have sawdust bucket composting toilets, and use pallet composting bins (about 4 ft. cubes) to compost outside. We have just two adults as well, and a toddler, and we fill up one bin per year (we also both work at home, so almost all of our waste stays on site). It takes two years for humanure compost to be safe for use on vegetable gardens, but it could be used sooner on flowers or trees. We always use ours on trees and shrubs anyway, and it's nicely composted a year after filling the bin. A 4 ft cube is way more volume than a barrel composter, but if you're talking about those ones you rotate, they would compost faster. However, the liquid from the buckets would run right out of them, and you do need the pee as part of the compost to balance the high-carbon sawdust. Also, there is a smell around our compost bins for up to 3 or 4 hours after dumping the bucket, which isn't a major problem outside in the country, but might be objectionable in a garage... As for the temperature, the compost will generate plenty of it's own heat once you get enough volume built up, but I think you do need a decent size bin for that. To be completely honest, it's not something that I would attempt, but if you could figure out a workable bin system it could be done.
DH, 7 mo old DS, and I are about to move into a cute condo with a two car garage. We're planning to live there for a year or so while we find a farm to buy, and we don't have a car, so the garage will be empty aside from whatever craft projects land there.
I've been dreaming of having a sawdust/composting bucket toilet for ages, and I'm hoping this might be the time. I'd like to set up my toilet in the main house and a compost barrel in the garage, and use the same compost barrel for household scraps. Is this insane? Can I compost well in a cool garage? Will smell (of the compost) be an issue?