Just hit the same issue. Global lock doesn't seem very useful and has a fundamental flaw of not actually preventing conflicts between branches (until "traveling locks" are implemented). We were also planning on branching one way and never merging back at some point, for porting reasons. "Traveling locks" might make that impossible. I suppose we'll look into pushing the branch to a different repo, but it is always nice to have the option to merge later, in case we didn't plan ahead perfectly.
Ideally locking would be branch specific. For our use case, I'd rather have merge conflicts that I manually resolve for binary files than global locking. Locking is nice for our main branch for day to day work, though.
thanks