This requires some support from Round Cube in order to figure out the correct mapping. I'm not sure whether such a plugin exists--SquirrelMail has a virtual hosting plugin for this purpose. Usermin has built-in support for Postfix virtual maps and Sendmail generics tables.
Ah, looks like it can handle it:
http://trac.roundcube.net/wiki/Howto_Config#ConfiguringforVirtualUsersI think that's the same format as in /etc/postfix/virtual.
If it works, let me know, and I'll add it to the default installation configuration in Install Scripts.