It looks as if someone has done a lot of the heavy lifting towards making an operating system for the 6502. The project is DOS/65 by Richard A. Leary.
This could save a lot of work. Richard chose to have a lot of compatibility with CP/M and I've got a lot of time already invested with the Multicomp CP/M systems.
Not sure how tough it will be to port the DOS/65 system but it has to be a lot less than writing an OS from scratch.