How do I influence which addressbook application is run when I am making modifications in split screen mode?
Go to the split screen preference menu (tap on the paper icon in the top left corner on the screen). Select "Preferences". In the bottom right area of the window you will find the text "Addr CreatID: addr".
To use a third party address book replacement change the creator ID from "addr" (for the Palm addressbook) to the creator ID of your software.