I don't know of any 68k backend and would certainly appreciate it if somebody took up the task! I'm working on a frontend for compatibility with the old Amos BASIC on the Amiga and was going to have to start a new backend for the classic models.
That being said, I don't think 3 or 4 weeks would be enough unless you're really good with both 68k Assembly and LLVM's IR. The IR internally represents its code as a 3 address instruction set as far as I know from reading the backend documentation for some older releases of LLVM. The TableGen application helps though.
Let me know what you decide on and I might be able to take some time out from the frontend I am working on to help you out with the 68k backend.