I'd expect Plough Lane to be available. The Women played there yesterday (and currently don't have any future matches scheduled for PL, although that will change at some point) and of course the next three London matches are at PL. I'm assuming a home play-off would be on Sunday 1st October. That is the day after an AFC Wimbledon home match, but a fortnight before their next home match. So don't see the football club having any issues.
What is not clear is whether London want to host extra matches at PL. The attendance figures suggest they alone don't cover the match rental. No Cup matches were played at PL after the Whitehaven match when we were all seated in the Hospitality seats. A Play-Off match should be viable at PL especially as I'm sure whoever the visitors are would bring a good following. If it were deemed not viable then it's hard to envisage London can remain at PL for much longer.
Apart from AFC Wimbledon Men, AFC Wimbledon Ladies (minimum four matches this season) and London Broncos, only the AFC Wimbledon U18's play at PL. Any Youth Cup home match is played at PL, but to date no other U18 matches. There are some one off's like the Womens League Cup Final, Womens International matches are behind closed doors men's football (often involving Brentford B team!)