How to allocate project transactions multiple time?

I have a client with the following requirement:

When they process a proforma invoice there can be a delay of 2 to 3 weeks before the customer agrees on what they are going to accept on the bill, and they can process the AR bill, which may be over a period end. They want to be able to record a type of WIP entry, debit WIP and credit Accrued Revenue, for the value of the Proforma invoice so that the revenue is recognised in the period in which the proforma invoice is raised.

I have looked at how this could be achieved and cannot see how to achieve this. The Allocation Rules essentially cover the posting of WIP when project transactions are created and are then reversed when the when the AR bill is generated/released.

I tried to create two tasks:
On one task, I create an allocation rule to allocate Cost to WIP.
On the second task, I created an allocation rule to allocate WIP to Unbilled/Accrued Revenue, and Billing Rule to select from Unbilled Revenue and post to Revenue.

But it seems that when I run allocation once, it works fine. when I ran allocation the second time to move from WIP to Unbilled, no allocation transaction is generated.

How can we allocate project transactions multiple times, or is there any cleaner way to achieve the above scenario?