I know I can create and upload custom prompts, but is there a way to upload custom mailbox greetings (both to 7030 & MP20s)

As far as i know there is no way. I wish you could.
The work around was to use a menu to pay the getting, then send the call to a mailbox which had a couple of seconds.of silence recorded as the mailbox greeting, this stops the "mailbox 3501... Recording" message being played.