I'd suggest just keeping CFI signatures in a paper log book. Current FAR's require written logbook endorsments (even on a McDonalds napkin is ok) but there is no provision for 'electronic' signatures in the FAR's. I still keep the name of the instructor (or student) in the remarks section in LogbookPro, but definatly keep all endorsements and such in a paper log. (This is from an AOPA interpretation that came up from a question at a recent flight instructor refresher course I attended.
I suggest purchasing one of the leather binders for your Logbook Pro reports from the MGOent collection at The MGOent Store. This way you can print out your electronic reports and also keep a separate page in the back for your instructor to sign. You can create your own pages using a simple word processor such as Microsoft Word. Create the same 'additional' pages that you want and just put them in your binder just as a hard bound paper log has it. If you purchase the full-sized binder (98-003) this will give you full flexibility as it handles 8 1/2 x 11. Lastly, the binders are on sale for up to $30 off until July 31st, so this is also a great time to take advantage of those.
I will consider adding a report to printout blank endorsement pages in the upcoming version 1.7. I think this may help this need for handwritten signatures, in combination with the leather binders, I think we have the answer.
Thanks for using this forum so others can learn from your question.