It didn't take me long to figure out that the Sony Ericsson phones (judging from this, at least as far back as the T28s, but the best documentation I have is taken from the R320 manual) support adding extra menus to the "Accessories" menu by sending AT command codes from the Mac.
The base command codes are:
AT*EAM="<main menu title>" AT*EASM="<menu name>",<bitmap state>,<menu index>,<menu item count>,"<menu item 1>", ..."<menu item n>"
*EAMI:<selected option number>
All in all, it is a reasonably simple protocol to deal with, and if I ever hang on to a T610 (or similar) phone long enough, I'll update my PowerPoint Remote applet to deal with it (Veta Universal apparently implements the phone side of it on the Series 60, so I might be able to do something with my 7650).