I've only built one model with undercamber, and it doesn't have much (BUSA Bristol M-1). I had the same concerns, and this is what I did, worked great, but I don't know how necessary it was, as I didn't try covering the regular way to see if the fabric would stay.

I used Koverall, and prepped the airframe the usual way, fine sanding, applied two coats of nitrate dope to the frame everywhere the fabric would contact. I applied Stix-It to the perimeter of the wimg (leading and trailing edges, aileron bay), and also to the tops of the ribs. On the underside of the ribs, I applied Stix-It at half inch intervals. Then I applied the fabric to the underside of the wing, tacking it to the ribs from center to each edge, smoothing out wrinkles by gently tugging. I then took medium Ca and applied it to the fabric contacting the ribs, in between the spots of Stix-It. I covered the remainder of the wing as usual. When I used my iron to shrink the fabric, the Ca had it well bonded to the ribs, so as it shrunk it did not pull free from the undercambered surface. I then taped the ribs and edges and finished the Koverall as usual.

If I were building something with enough scale fidelity that it used a truly undercambered wing, I think I'd stitch the fabric to the ribs.