So I went down the list of PeerTube clients in F-Droid, and the only two that let me pick my instance / login are TubeLab and P2Play. They look nice but there's a problem: they don't understand login with 2FA, and I don't really want to drop 2FA as I always use the extra security whenever it's available.
Any other Android client you're aware of that would work for me?
In the meantime, I'll open a feature request for those two apps.
EDIT:
Uuh nevermind, I answered my own question: when I went to open an issue with TubeLab, the repo says it's now part of FediLab - which, inconveniently, doesn't come up when you search for PeerTube in F-Droid.
I installed FediLab, and lo and behold... 2FA!
And with that, voila: I'm logged in:
Brilliant!