If we could get the equivalent FskDocumentViewerAdobe.so from the 900 (which does not display page numbers I think, and also supports full justification) then we could try and replacing it for the one on the PRS505. I would try this myself but I do not have a 900, and I don't know how to get the epub viewer out of the firmware.
I have also checked out the ADE SDK
. You have to apply to be licensed, and I suspect Adobe would not license it for this purpose (changing the firmware on an existing device that is not open source).