Your idea sounds CUTE!!
How about getting a pair of slippers from the dollar store and cutting them so the top part can sit on top of her hooves to make it look like she's wearing slippers. You could also go to a halloween or joke store and get those BIG sunglasses and take the lenses out so it looks like she's wearing glasses. If you decide to go with the old lady costume you definitely have to post pics it sounds sooo cute!!!
I have a halloween show this weekend and at first I was going to dress Phantom up as a Zebra and me a safari person but I changed it to me as a crazy veterinarian who chopped up then sewed her horse back together lol.
This is what I anticipate he'll look like(amature edit painting lol)