A. Melody menu | Mute melody in middle choruses [checked]. This is a quick way to do it, but this is not a song setting and is not saved with your song.
B. Bar settings - click on Bar 1 and press F5: select chorus 2, set the Melody to 'Mute'. Then select chorus 6 and set it to 'back to normal'.
I've just realised that, as I'd learned two ways apparently to do this, I didn't properly recognise A. above. It does clearly indicates Melody Menu -> Mute melody in the middle. Probably that [checked] threw me.
My understanding now is that there are three ways to do it:
A) Melody menu -> Mute melody during...
B) Bar settings as above
C) In the Select Soloist dialog, middle right "[X] Mute melody in middle".
I was using C, I've changed to using B, I guess I can use A any time, but need to use it at the time I want it.
FYI:
I've just tried the file I saved _before_ I set F5->Mutes and it mutes at least the second chorus, I'll presume for now that it unmutes the final.
I've just tried the version I saved after setting the F5-Mutes and it mutes as above.
Possibly I should simply do that every time I use BIAB for practice.
But I can't presently escape the conclusion that, on this song at least, the F5 mute is simply not muting.