There already is a way to change the theme at the desktop level for the onboard keyboard. From the panel indicator you can select preferences and then theme. This sets the desktop onboard theme and also the onboad which appears at screen lockout. The greeter is the only onboard keyboard which has the default theme statically set. On my 12.10 system there are 3 places where the onboard keyboard appears: 1) greeter. 2) desktop invocation by launcher, indicator, shortcut, etc. 3) screen lockout with onboard at bottom of password screen. I am guessing you can read the existing gschema setting for onboard at greeter screen and apply it. So maybe ability to set onboard at greeter is not needed.
There already is a way to change the theme at the desktop level for the onboard keyboard. From the panel indicator you can select preferences and then theme. This sets the desktop onboard theme and also the onboad which appears at screen lockout. The greeter is the only onboard keyboard which has the default theme statically set. On my 12.10 system there are 3 places where the onboard keyboard appears: 1) greeter. 2) desktop invocation by launcher, indicator, shortcut, etc. 3) screen lockout with onboard at bottom of password screen. I am guessing you can read the existing gschema setting for onboard at greeter screen and apply it. So maybe ability to set onboard at greeter is not needed.