I'll definitely take a look at the guide to see if I can make something using my normal Kickassembler setup. I don't know if it's some kind of API, but sounds like fun to find out :)
i haven't tried it in a while, but if anyone is interested i have a cross-compiler workspace setup here: https://github.com/woodrowbarlow/c64os-example-app
(the include files probably need to be updated)
i also worked on a python package for bundling/unbundling "car" files (a single-file bundle-archive format used by c64os): https://github.com/woodrowbarlow/c64os-util
I've written elsewhere on how Commodore missed out on an opportunity by not including GEOS on ROM on the C64C. <https://np.reddit.com/r/c64/comments/1c6ti2n/this_is_surpris...> I can only imagine how doing so with C64OS would have changed the world.