We've actually been working on something like this for San Francisco (and we plan to expand onwards after operating here): http://www.takehitch.com

The municipality tends to work a little slower here (chatted a bit with SFMTA in the past), so we're starting with a model that just focuses on private cars. But the fundamental point--dynamic re-routing to create custom routes where people can ride in the same vehicle--is very much in the spirit of what we're doing.

We're still in a pre-launch phase, but check out our iOS app: http://www.takehitch.com/download