This sounds great to me. It doesn't require the introduction of a new option, and it'd be easy for downstream tools like odkmeta to implement. The convention in XLSForm seems to be to use underscores rather than hyphens in names, so hopefully hyphens in group names are not common. What replacement character do you think would work well? Also, do you know offhand how Briefcase replaces colons in names? (Those aren't allowed in Windows filenames I think?)
Perhaps I should also try to summarize this part of the conversation for the rest of the Central team, since one goal for Central has been to export CSVs that match the Briefcase export.
I'm actually close to releasing a new version of odkmeta for the first time in a while, so I'm in a good position at the moment to implement any changes to CSV filenames.