I agree that some of the conventional methods that let the pollen fall out on its own, don't work as well with reversed females, as they do with regular males.
The good news, is that you really don't need a lot, so what you can do, is wait until the flowers are open and falling off when you shake the branch. That ensures they are mature. Shake the branch around over a plate, and then hand pick some stamens out while throwing away the remaining parts of the pollen sacs (they contain moisture). Let them sit out for a day or so to dry out, and then tap them with a credit card edge, or something thin, but not so sharp that it cuts them. They should shoot out little puffs of pollen. It's tedious, but honestly, using tweezers and both hands, to do this to each individual stamen, is more effective than trying to smash them all up, because if there is any moisture, it will cause your pollen to clump and be ruined. If you do this over glass, you can use a razor to push all the pollen into a small pile. Even a very little bit will get you hundreds of seeds.