Actually I don't think the handling of ABS_FILE is hidden. I don't have a CW installation here (just installed vista), but I guess this var is passed via -env command line option. Check the echo command line box in the burner preferences in a CW project and build once. You should get the full command line as CW is using it, and using the same command line from dos/make, you should get the same result.
For a makefile based use for the burner there is even a simpler way, check the burner documentation. The burner supports to pass in the content of a bbl file directly as command line argument, not sure of the option and the exact syntax though.
Daniel