The better option would be to pass all source files to update in one invocation of dmd, so it won't get slower than a full rebuild, but this has been plagued with linker errors in the past (undefined and duplicate symbols). If it works, it could identify independent group of files which you now separate into libraries.
Aside from linker errors, there is one more (minor) issue with this approach. If there are multiple source files with the same name and there is no -of flag, DMD will generate an object file for just one of them. This could be worked around in rdmd by symlinking or copying files, but I think it would be better to fix it in DMD and use module.name.o instead of source_file_name.o for object file names.
