Great, thanks, @pianoleo – glad this is a limitation and not me being dense. My workaround is to have a3 after a system break and then switch back to unison, a2, and a3 where needed. Works fine:
Unfortunately, when I condense the divis the brackets will disappear (which is a known issue: Condensing Line Disappearing - #23 by Mark_Barden). So my final solution is to simply not use built-in divisi but to use text:
