I recall Bbucko mentioning something about doing this...perhap's he'll chime in?
As Hilly notes above, it's something a culmulation of several hours' fisting play. Much like punch-fucking, it's not anything that happens after a 30-minute warm-up.
There were two points that worked really well in my favor:
1) He was actually a bit shorter and slighter than me, so there was no tangle of gangly legs involved; and
2) He had a 30" length of neoprene (about 1/2" wide. which we wrapped very tightly into a sort of cockring/ball-stretcher arrangement.
The neoprene not only stretched very tightly, but was easy for him to clean later (unlike leather, for instance).
As far as positioning went, he had knees and head on the mattress, ass high in the air. I fucked him from behind, inserting balls in first then pushing in a half-hard dick. Several minutes of fucking him and I was all inside and rock-hard. We repeated this several times that night, in between the more expected bouts of fisting, fucking and WS. It was really hot.
I've jerked off up guy's asses (full-hand penetration plus hard cock) many times, but this was something of an odd, unique personal best.