No women Aikidoka ever told me not to bruise her. I just don't like when a woman has bruises.
You do realize that you've completely changed the source of your problem with training with women, don't you? Before you said that women don't like bruises. Now you're saying that YOU'RE the one who doesn't like them. It's okay not to want to bruise someone, but why doesn't that extend to ALL your training partners?
It is very easy to bruise when applying yonkyo.
So, my question is:
How to apply yonkyo on women?
The same way you apply yonkyo on men. Don't worry so much about grinding your knuckle into the nerve. Concentrate instead on controlling uke's center through the elbow. The nerve is just grins and yuks. Best compliment I ever recieved in the dojo was "You're the only person who has ever done yonkyo on me where it was effective without leaving a bruise."