Personally, i would be honest with them and say you cant make it.
I had a natural birth but had to have an episiotomy. I had other issues too but wasnt able to stand up for long periods for over a month. I think you will be putting too much stress on yourself and your body to be honest.. even if everything goes well with your labour.. you will be still required to stand for some long periods and that couldnt be good.
Best of luck with your decision.