I finally got around to use the mediamanager in a custom form field created by a plugin.
Not sure if the proposed changes in another thread (about securing the access to the mediamanager) will break that.

If anyone is interested, I included it in the bureaucracy plugin from A, Gohr.
I also put a few other additions to the plugin [date picker, link and phone fields, constants, a few more options...]
Most of the plugin is now in the helper part so it can be reused in other plugins. Basic syntax plugin works as before.
You can find this version at:

http://meteokid.ca/wiki-plugins/bureaucracy.tgz

Please note that it is a preliminary version, lot more work needs to be done, mainly to secure it.
If you can help that would be great!

Not sure if A.Gohr wants to include some of these in the official bureaucracy plugin... would be nice

Thanks!
Stef