Since no one has chimed in on your request I'll give it a shot.
Assuming the PMI insurance is a fixed amount use a split transaction for your mortgage payments, allocating one of the splits in your mortgage payment to the PMI account. In the same way you can split the property tax and insurance amounts into their own accounts. When you wish to see total escrow not including the PMI I would run a report using property tax and insurance as inputs.
If for some reason the PMI insurance is a variable amount then it is more difficult to set up an accurate reminder for future transactions but you could still estimate it and then edit the transactions as they occur to account for it.