I finished porting my old Bezier enveloping function from Batik/Java to Inkscape/Python. I'll call it "Bezier Envelope".
It takes an arbitrary path (the "letter") and a 4-sided path (the "envelope") as input, and then attempts to squeeze the letter into the envelope by rearranging all anchor and handle points of the letter's path. That means the path is not broken down any further, so a rough letter might not fit into a detailed envelope. Otherwise, it works pretty well.
A detailed description and lots of comments are included with the Python script. It would be cool if someone could help to improve precision and usability... Maybe some of the PerspectiveObject fans?
The most important improvements would be to find or a better bounding box function (this must exist somewhere), and to convert lines and quads to cubics (probably exists somewhere too), because currently the extension only works with cubic paths.
Check it out if you like, and tell me what is still needed to include it in the regular Inkscape distribution.
Cheers to everyone, Gerrit