John. Glad it helped a little at least.
Have you looked around Dosadi's site? They also have a package (under Products) called CTwain, which is a library with source for use in building TWAIN apps. Their licencing is very generous in that it "may be freely used for non-commercial purposes". It is designed for exactly your purpose I think.
They also have a support dept. which, going by my one intereaction with them, seems very knowledgable, efficient and flexible. A rarity these days.
I'd suggest you contact them, tell them what you are trying to do, and maybe they will be able to give you some pointers. Good luck
Cor! Like yer ring! ... HALO dammit! ... 'Ave it yer way! Hal-lo, Mister la-de-da. ... Like yer ring!