I don't think there is any problem at all with having one database per user, indeed, I already do this in some of my projects. The database proxying makes it almost trivial to implement in Rails.
If having too many users on one server turns out to be a problem, move half of them to another, easy.
Migrations, i.e. changes in properties can be a bit more complicated as you'd need to go through each user's database and perform the modification, but its not too difficult.
Couch Rest Model should handle changes in the design documents for each database. A cache is used to avoid performing loads of GET requests to check for updates, but it can be disabled as long as you know your design docs are all up to date.
Give it a try and let us know how you get on!