"My Q is along the lines of- if she says she's fetish friendly and you want to try X, Y, or Z, or you want a specific outfit or roleplay scene, when do you bring it up?"
If she is actually fetish friendly and this really isn't a graphic or sexual detailing of your fantasy, then a suggestion that you have a fetish related request from the beginning is a good start. Then:
"After she responds back to you?" - yes, it is assumed that she is now prepared for you to bring this to the table as a query. Be polite and approach this as a general outline to see if this is going to be in her "no fly zone" for fetish or kink play. Anyone who tells you that NOTHING bothers him/her is a liar or completely inexperienced (or just really bad at what they do). Everyone has a hard limit. Better to know where the squick factor is BEFORE you invest a lot of time and hope.
"After a firm appointment is arranged?" - Possible but not best approach unless you are flexible about your fetish needs and can live without them. See "squick factor" above.
"When you meet in person?" - Oh HELL NO. Unless your fetish involves absolutely no real participation on his/her end, no preparation or clean up of the space whatsoever, no props or specific types of clothing AND is so benign that you are basically staring at each other from across the room without touching - then do NOT wait until you show up to mention it! Even if you had a tickling fetish (how much more benign can we get than that?), there are still considerations around this that require the provider to know ahead of time for this to be possible for you. What if the provider finds NO pleasure in being tickled? What if there is a noise issue with the neighbors between certain hours? What if the provider has some kind of negative association with being poked and prodded against her will? How awkward will this be to arrive and discover this AFTER the time has been committed to and this is all you really wanted?
This is even more important when discussing certain types of role play because some things require mental preparation more than anything else and other things are absolute "no fly zones" for some folks, no matter who is asking.
So get it on the table early to see if this is even possible. Get into the details after approval and before showing up. At least, that's how it works for me.