My group has been developing a family of applications somewhat along these lines
for several years.
ScanScribe (http://www.parc.com/scanscribe) is a document image editor. You can load a bitmap and easily type into it, select and rearrange material, and cut and paste image objects from elsewhere (e.g. your signature image). Our mission is perceptual document analysis, so we play tricks like make white background transparent so you can position foreground material without occluding things behind.

We haven't specifically designed ScanScribe to recognize fields on forms, but if this were in sufficient demand, our image analysis tools would enable us to build a reasonably effective "tab to next field" function. The other bells and whistles you mention could follow easily.

As a free download that runs in Java, this is a research prototype, not a product! So don't expect a polished application. But I myself use it for filling out forms, among tons of other things structured bitmap editing is useful for, and it works quite handily.
-Eric Saund