Have you though about making a balcony box? get one of those under the bed storage boxes, pop some drainage holes on one side (usually balconies have a drainage slope away from the building so place them on that side) put some soil in the box and a few pieces of sod to cover the top. You would have to keep the sod watered to stop it from dying but its only outside the door so thats not too hard, and giving a it a water rinses the pee off the grass. You could also put a pebble layer and put like burlap or somthing down before adding the soil for better drainage.
This would keep with the theme of peeing OUTSIDE and prefferably on grass, most dogs try and keep it on the grass and don't like going on soild surfaces as much.
You could set that up and then add a ringing bell so that when she wants to go out to potty have a bell to ring with her nose to indicate to open the door.
This is the set up my dads GF for her little dog when she was in a apartment and it worked just fine