I can code in C and would be glad to help. I am still learning the structure of pluto so there is still some fuzziness on how all this hangs together. Right now motion is recording continuously and recycling the images. I haven't delved into how motion gets its config file and how that config file is generated. So it sounds like you plan on intercepting it and adding/removing parameters outside of pluto web forms?
I think that motion-wrapper code will be pretty straightforward for you as C programmer.... motion.conf is currently hardcoded and generated when motion is started. I think that template based approach (look in /usr/pluto/templates/ ) would be much better, but also custom parameter feature is also important - user will be able to determine custom lines in threadX.conf - that motion-wrapper will catch and put into config files....
There is also one interesting thing. Motion 3.2.3 has http API defined - it gives you additional controls - and that could be included in motion wrapper:http://www.lavrsen.dk/twiki/bin/view/Motion/MotionHttpAPI?CGISESSID=bcd3a609604528b7300e3b0271018cd8
will be glad to help further if needed,