I too have iris and daylily pollen stored since I pollinate and grow the seedlings. I have also pollinated hardy Hibiscus but the only seeds I got was from a white variety. All were seed grown.
I never sowed the seeds so have no idea what the seedling would look like. I don't see any reason why you can't store Hibiscus pollen. All my pollen are in vials then stored in the freezer. But first let the pollen dry over night on a paper plate or napkin. Before freezing, remove the stamen since it is moist.
Be sure that the pollen is ripe when you harvest it. It should be powdery.
