Taking advantage of having a T610 to fiddle around with (and having done similar work before to develop my PowerPoint Remote applet), I decided to take a look at how Romeo and Sailing Clicker work.
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>"
There is also AT*EAID to make the phone display an input dialog and a notification option (which might be how Romeo manages to control the Mac mouse cursor), but the base reply from the phone is
*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).