I have just tried to do this with a wm5 device, and no not possible.
causes your pda to become a white brick... ie white screen of death.
used both registry hack and sktools to set default sip to ribboard causes WSOD
If your using windows mobile 2003 the registry hack and sktools setting default sip to ripboard works, but I wouldn't try it on a wm5 device, unless you've got a good stable backup to restore

:)