I have done it.

I hired my IM for support at home for as long as possible before heading to my private hospital with her where she acted as a support person. As Kelly said they aren't allowed to do any of the medical stuff once you get to hospital.

I found though that the hospital midwives stood back and let my IM do her thing only interjecting to listen to bubs heatbeat and do any BP checks and that sort of stuff. I spose it will depend on the day and the midwife you have at hospital as to how out of the way they will be......they might feel a bit threatened or they might see it as a blessing because they won't have to be with you so much, especially if they are busy.

I was fully up front with the hospital when I booked in telling them that I was bringing my own IM which they noted down and it helps if your IM has a bit of a good repore with the hospital too. My hospital new my IM and she had been there before in a supporting capacity.
My OB also knew her too, so I did shared care with my IM and my OB, alternating appointments between them. The OB you can claim on medicare but the IM you can't so it is a full out of pocket expense.

I was VBA2Cing and I totally believe that if I hadn't have had my IM I would have ended up with another CS. So in my opinion they are money very well spent!!