I ran into an issue using NVC when setting --work=top and setting -L /path/to/dir/containing/top thinking it would take the -L command into account when searching for the work lib, but it does not for some reason.

It actually used to work like this a long time ago but I changed it because it seems more important for files to be written in a predictable location (e.g. if you already had a library called top in the search path but forgot about it NVC will silently write its output into an unexpected location) . For scripts and test harnesses the most robust method is to specify the location of all libraries explicitly with --work name:path and --map name:path.

Is that separator still supported on Windows or is there a hard switch between the two depending upon the OS?

Yeah it's not documented but Windows accepts both : and ;. This is mostly to work around the MSYS2 shell silently replacing : with ;. From cmd or PowerShell --map foo:C:\x\y\libdir should work just fine and is unambiguous since we can split on the first :.