Hm.. I haven't tried it, as I don't think I've had a customer with Embedded VM that would really need to.. but shouldn't you be able to transfer a line/DID to a shortcode to edit the AA greeting? For example, I think the default for AA #01 is *8401 for the shortcode. Just point a number to that code and it should work I would think.
And for personal VM boxes, you have to make sure to set up a password for the voicemail box, and then point a number to "VoiceMail" under destinations or *17.
You should also be able to do all this with options off the Attendant as well if you're using analog lines with no extras to spare.
Someone correct me if I'm just talking out of my proverbial behind here.