I would like to build based on a CVS tag rather than on HEAD or on a branch.
The tag I use is movable and corresponds to the current baseline plus
candidates for inclusion in the next release.

I have tried to use the tag name in the branch field but I believe the -D
<timestamp> argument, that is automatically supplied, causes -r <tag> to be
interpreted as only a branch.

I suggest a separate box called Tag below the current Branch box but there
might be other ways to implement this.