Hey Richard,
We have looked into doing this but found that there is an SDK limitation with converting Estimates to Invoices. We did find that using Method we could copy all the items on an Estimate, and inject that into the invoice table. The problem with this is that QB will not know that the conversion has happened, it will look at that Invoice as a new Invoice, not as a converted Estimate.
In Method you could create filters and fields to show you what ones were converted, but that data would not be available in QB.
Hope this gives you some idea's.