The feeling like you're gonna pee sensation is just that...a sensation...a sensation which quickly turns into a pleasurable orgasam if you just allow it to. I think that maybe if the girl is actually working on trying to ejac. and working through the sensations, maybe she might pee a little in confusion but if that happens, I wouldn't consider it a huge problem...quickly clean up and start over.
I really don't think that so few (less than 50%) women are capable of this...I think it's more of a fear of peeing...or just holding back. I didn't start to ejaculate until a few yrs. ago, but I have the same body that I've always had...so in hindsight, I'm sure I was holding back for many years.
I've said it before...fem. ejac. (at least for me) is as much mental as it is physical. ie. rolling on X I could not pee if I wanted to, but I absolutely loved having sex while rolling and had some of my wettest orgasams while rolling. I also usually don't ejac. when I masturbate...unless I'm really getting into it and letting my mind go to fantasy land.
There's also the issue of hydration...most people don't drink enough water. If a girl is gonna ejac. she's got to have pleanty of liquid in her body. There have been pleanty of times where we've had to stop to rehydrate...my mough will become completely dry and I get dizzy and cold. Down a quart of water and relax a bit and I'm ready to go again.
BTW...Seymore Butts has the best squirting videos. I forget which ones I've seen, but in some of them he goes into detail about how to stimulate a girl to get her going good. Some of them will be guy on girl stuff, while others are girl/girl or group masturbation videos. There are a lot of fake squirting videos out there...his videos are real and I happen to like his personality...he does the directing and often talks in the movies.