it would steepen the geometry and make it really twitchy
It wouldn't alter the geometry as the wheel contact points and the relationship to the dropouts are identical if both wheels are the same size - just a bit lower to the ground.
As you say, the BB will be lower so there'll be clearance issues, plus it's possible
that you might have tyre clearance problems if the chainstays/seatstays are shaped around the tyre.
Good point! Only changing the front wheel would screw it up.
Surely it shouldn't make a difference for the rear tyre, The frame needs to go from the 68/73 BB shell into the 135 rear spacing. So even if the chainstays were kicked in there would still be enough clearance as a 29" tyre fitted in the frame. So if anything wouldn't you be able to fit wider tyres??