OP, your question presupposes that prospective families will want your input. As a longtime HM who has talked to extending families and rematch families over the years, I can just tell you that this is not always the case. We have had prospective families of rematch and extending Aps call and simply verify that someone in fact did x y or z activities with us. We have had prospective families call and ask useless questions that reveal nothing about the kind of person or AP someone is. We have even had a family call and argue with us when i tried to share about why we weren't keeping an AP through his term. I have learned that the best thing to do is to put in writing only the low inference stuff - what the AP has done for how long, and to allow HFs to ask questions about the high inference - what did she seem like and what did we think of her work. In the end, there is a lid for every pot, and hopefully your AP will find her pot. The only thing you can do is answer every question asked of you honestly, and if people don't want to ask (good) questions or don't want to believe your answers, then there's really nothing you can do about it.

As a side note to your comment about California, it's hard to not to feel a little bitter about an AP not wanting to extend, even if you wanted her to go. It's kind of like a break up when the other person breaks up with you before you can break up with them. It's a bit of an ego blow, but, trust me, you are going to be dancing when she leaves the house!