I doubt there will be any MM play testing; I’m pretty sure they sorted out what their plan were back when they put out Monsters of the Multiverse. I‘d bet that book is more than halfway done already.
As for the DMG, there's only so much of it that actually needs play testing to begin with—all the DMing advice and lists and tables are likely well underway. All the really need to test are things like Bastions, any updates to Downtime, Encounter Building, maybe some magic items, possibly a handful of rule options. They can easily work that into a few rounds of play tests and still have time for a 4th quarter release.