The on-screen keyboard is a screen in orbiter, therefore it must be edited in HA Designer. The designobj you want to edit is 3292, the generic app full screen designobj.
There, you will see a button, for the keyboard, there should be a Goto DesignObj in its onActivate, to go to another designobj, this designobj is the Virtual keyboard.
This keyboard needs to be re-made...that is, each little designobj in it, needs to be deleted, and then replaced with new designobjs that send their keystrokes to another place...probably -203... but I am not sure. There will need to be experimentation here.
The whole mechanism to send keypresses to the display was literally a quick hack inserted by someone at pluto, and needs to be rethought.