I think I'd be fine with that, it just seems minor enough that it would look out of place if she got custom art. I'd personally just limit it to the zanti content, like her asking you where the farm is and mentioning looking for a job, with an image of her being milked for later, to make the whole thing more consistent.
I'd still have to ask you to either donate 25$ or join that tier on patreon, since it wouldn't be fair to the others who donated to get something specific added to the game. I'd understand if you weren't looking forward to spending that money on top of commissioning the art (not that any of this is mandatory).
What would she look like, and do you have a name in mind?
If you just want me to name this NPC how you want, and that's it, I can do that, no need for donations. It seems like a little scene that would fit the cowgirl path anyway.