Done it a fair bit. Legs over head and jacking it straight in.
Now I have a brilliant gag with a cock sheath with a tube running from the sheath to an inflatable rubber gag. I had to adapt the tubing a bit as it was really long originally and also quite flexible so it kinked up a lot. I replaced it with slightly larger diameter, and thicker tubing to help it flow, as well as reducing the length to speed up the flow.
Strapping myself up with my legs held high, vibrating inflatable plug in my rear and a vibrator taped to my cock sheath. As the vibrator does its job the cum flows into the tube and begins trickling down. The first taste takes a few minutes to come through, but depending how turned on I am I may cum quickly again, increasing the flow and ensuring that I get a decent amount in. No chickening out here!