I have heard this as well, so I was interested to see answers or at least look into this myself. I haven't published on this subject, so I can only rely on other sources.

I checked Rosemary Ellen Guilley, The Encyclopedia of Witches and Witchcraft (1999), and she makes no specific reference vaginal absorption. In numerous entries she writes of rubbing the ointment on the broom handle and on the witch's body, but she never "makes the connection" as you suggest (and I have heard). Clearly, certain tissues would be particularly useful in absorbing hallucinogens, and clearly these drugs were part of at least some ceremonies - either in reality or as imagined by authors and by those who prosecuted the Inquisition. The phallic nature of a broom handle suggests an obvious means of application.

The problem we face, as historians, is that so much of what was written about witchcraft was based on what so-called authorities imagined this anti-Christian sect to be doing. Were there people who used traditional non-(or pre)Christian magic for good and/or ill? Yes. Were various drugs part of these and other practices? It is hard to imagine that this was not the case. Did the Inquisition prosecute innocent people? Almost certainly yes. Does all of this mean that assertions that witches did things one way or the other are accurate? There is no way of telling, in general.

I am surprised that Guilley does not address your specific question, but if she did, I suspect she would say that there is no good evidence that this was the case, nor is there evidence that it was not the case. There is simply a lack of good evidence that is not tainted by the wild imaginations of the members of the Inquisition and of their sad victims who were too often willing to say anything to make the pain stop.