c64os is a very cool project. it's a modern operating system that pushes the bounds of what's possible on the C64. it's commercial software, available for about $60, and has a full application SDK.
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 :)
the author of c64os uses turbo macro pro, so all the include files are in tmp format. there was an un-official kick-assembler port floating around, but i don't know if it's still updated.
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