Dihedral really has little to do with pitch either upright or inverted. I regularly fly 3 and four channel old timers inverted and they have very flat bottom airfoils and a bunch of dihedral. most people don't put in near enough down elevator at the right time to hold the nose from pitching down. Put in the down elevator at the right time and you can roll as slowly as you like.
ORIGINAL: warhwk
The dehedral will pull a trainer down very quickly inverted. I would suggest you start your roll after a gentle climb, say 20-25 degrees and let it roll around completely. Dont worry about rudder or elevator correction just yet. It might help if you increase the aileron deflection to speed up your maneuver. It really sounds like you are rolling too slowly.
Once you can roll exiting the same elevation as you started, you can practice feeding in down elevator while inverted.