Why not do it that way? Heck, girls do it in porn films whenever the Director decides he's got enough footage and tells the girl to fake a climax and the guy to follow it with a facial or whatever.
Yes, girls do indeed sometimes cum during shoots, but not usually at the end of the sex scene where it's made to look like it happened (sometimes, it's while the cameras aren't even running but the guy and gal are "messing" around; or, it's afterwards in the dressing room or shower or after the crew shuts down and goes out for a smoke, or etc.
So, a woman doing it like the originator of this thread suggested is reasonable/merciful/loving/etc???
Dave
www.davecummings.com