Hi, C.!!
First off, hugs to you and the girls! I would start collecting some old towels. Fold one up into a soft, cushy mat for her to sit on during time out. I would even go so far as taking off her clothing. Let her know in simple terms that she has to sit in time out because she is behaving badly, but since you know she will have to pee you are preparing for the mess. Maybe you could even sit her on a potty chair to eliminate the towel laundry. If she is being defiant, she'll probably pee off the towel/potty. If she does, I would clean it up with a simple, kindly, "pee goes in the toilet" reminder. (Get yourself some pet mess spray to help eliminate bacteria and odors) If she is doing it on purpose the last thing you want is for her plan to be effective with your reaction. I would also call her doctor and ask if they think she may need tobe checked out for any urinary issues or whether they think she is just holding back some pee from her initial bathroom visit to retaliate against the time out.
Maybe an alternative discipline would work better. I give time outs when there aren't easy consequences or the action is severe. Like if one of my girls spits, hits, kicks, etc. Otherwise, they get a consequence for their action. For instance, if someone throws a toy, that toy is put away for a few days. If one of my girls is splashing water out of the tub, then there are no bubbles and tub toys for the next few baths. This way timeout is not used as much. You may already do this method, since you didn't mention how often she is in time out, but I thought I'd mention it just in case.
This is a hard age, C.! You are a great mom and you show your girls so much love. Just make sure to hug her after each time out and I know you both will get through this!