When doing text input you might want to switch from fullscreen (GAPI-based) GapiDraw to windowed (GDI-based). Then you just write a normal Windows Mobile app so far as the SIP is concerned. Asking GAPI to only Flip part of the screen is asking for problems - how big is the SIP, how is Flip implemented on various devices, etc.
And then there is the issue of actually having a text entry panel - I haven't done any SmartPhone programming but rather than trying to recreate the capabilities of a text editor you could create an invisible (offscreen, etc) Edit control and have focus there, and shadow the display of the edit control into the GapiDraw surface (e.g. ask for the text & caret position, rely on notifications to update, etc)
Out of curiosity, how were you planning to handle character rendering? (A GDI font rendering function in GapiDraw would be a handy thing to have around.)