I've had great luck with Doncha's excellent domain mapping plugin since WPMU 2.8, but I've never tried it quite like this...
1) We have several sites at different domains (foo.com, bar.com, etc.com) that use Ruby on Rails and Apache (already working).
2) We would like to use Apache's mod_proxy to make foo.com/blog, bar.com/blog, and etc.com/blog all point to our WP VPS at wpblog.com/blog (also, already set up and working)
3) We would like these WP instances to be a single WP_ALLOW_MULTISITE installation that runs from that location and is domain-mapped to serve these domains as separate subdirectory installations.
Has anyone ever done something like this, using domain mapping?