I have not done so directly, but certainly in the evidence around the approach to negotiation that men and women take, clearly we have seen some difference: women seem to come from more of a position of gratitude, whereas men come more from a position of entitlement. That's been studied. When you look at how people conduct negotiation—and fundraising absolutely is a negotiation, in that I have a cause, you have money, let's make a deal—I think it would certainly not be an unreasonable issue to think that women may feel, based on the negotiation theory, a little bit diffident about asking.
Having said that, though, there certainly are some very powerful and successful women fundraisers. But that may come in part from their social networks that have absolutely nothing to do per se with any diffidence or not about making the ask.
Your observation, based on what you said you heard from witnesses, makes total sense to me. However, it's not an area that I have done any research on, other than knowing a little about negotiation theory.