Okay I *think* I've given this enough testing and it looks like it works, so here's what I ended up with:
addmap map01 dwango5.wad
addmap map02
addmap map06
addmap map07
addmap map12
addmap map13
addmap map19
addmap map01 brit10.wad
addmap map02
addmap map03
addmap map04
addmap map05
addmap map06
addmap map01 brit11.wad
addmap map02
addmap map03
addmap map04
addmap map05
addmap map06
addmap map07
sv_endmapscript "exec config/inc/brit11sfx.cfg"
// ↓
if sv_nextmap eq "map01" sv_nomonsters 0
if sv_curmap eq "map19" sv_nomonsters 1
if sv_curmap eq "map06" sv_nomonsters 1
It gets me what I wanted, but it relies on the exact maplist order, so things like map vote and shufflemaplist are right out.