rsync with overlay tree

[tomr]

I maintain a directory structure containing dirs and files that I regularly push to ~50 hosts, which are divided into 3 groups that have slightly different needs (minor mods in a couple of files).

So ideally I would have 4 directories:
    /path/to/sync/common/   <- common files
    /path/to/sync/group1/   <- group1 specific only
    /path/to/sync/group2/   <- group2 specific only
    /path/to/sync/group3/   <- group3 specific only

Then I'd run an rsync like:
    rsync -av --overlay /path/to/sync/groupN \
     /path/to/sync/common remotehost:

Thinking in terms of a list of files to be transferred, I would like:
- Anything present in common/ added to the file list; then
- Anything present in groupN/ added to the list, clobbering if applicable (regardless of mtime)
- The destination directory to show no sign of the common / overlay structure

I suspect I could populate the list myself and use '--files-from=<LIST>' but I would rather have rsync work it out if possible.  If all else fails, I will do the merge locally first.

    TEMPDIR=$(mktemp -d)
    cp -r /path/to/sync/common/* $TEMPDIR
    cp -r /path/to/sync/groupN/* $TEMPDIR
    rsync -av $TEMPDIR/* remotehost:
    rm -r $TEMPDIR

Thanks in advance,
tom