Hi.
This feature is planned but not implemented yet.
The reason is that at the moment the destination URL must be static.
In the next future will be possible to "group" part of source url, let users using in destination URL
I'm currently working on this feature and also trying to implement another Apache feature, the internal redirect (i.e. possibility to define a real alias without a 301 redirect).
Stay tuned.
Regards,
Luigi