Thank you Bob, that's a pretty good workaround, I tried it and it works for me. Thanks!
More generally, it's not 100% ideal, because it will prevent the user from regenerating the frozen track, but it's certainly useful, and it also requires changing the A-B structure of the song at least twice, but .. better than nothing.
I imagine (hope) that in future BIAB versions, in the F5 menu there will be something like a "switch to A" and "switch to B" option for each bar / track, just like for the volume automation options (mute etc).