How do planes respond aerodynamically when bringing them into inverted flight by rolling?
I fly such a plane.  My Laser has full inverted systems (oil, gas) and symmetrical airfoils.

You're right that when in upright flight the wings are producing lift in the direction away from the wheels and when in inverted flight the lift vector points in the opposite direction.

You're also correct that the wings are producing zero lift at 90 degrees of bank.

As for what happens during the roll, there's a smooth transition.  Assuming you're maintaining a constant heading, the lift provided by the wings will decrease to zero by the 90 degree of bank point, then resume in full at 180 degrees of bank in the opposite direction relative to the wing.  As lift decreases, the nose of the aircraft will typically decrease precipitously.  It's not an aerodynamic stall and there's no sudden onset.  It's just like rolling into a diving turn with the turn.

This is why trying to roll an airplane with no previous aerobatic training can be fatal.  That "simple roll" you tried can easily result in a >Vne dive.

There are 3 ways to not lose altitude during the roll:


As you suggested, roll fast enough.  My plane will roll a full 360 degrees in a bit over 1 second.  There's not much time to lose altitude :-)
Use "top rudder" (the foot closest to the sky) to lift the nose up.  At the 90 degree point you're using the side of the fuselage as a crude wing.  It works in the right airplane.  It requires an unusually large rudder and a lot of practice.
Anticipate the loss by pitching up a LOT before rolling.  You'll get more of a barrel roll, but it's still a roll.  


Please don't try this without competent training.  Stupid people die this way.