We are asking to be able to call any RFC on the SAP ALE connection, by making that RFC configurable in SI. IDOC_INBOUND_ASYNCHRONOUS should be the default RFC to use on SAP ALE connections, unless another one is selected. We can then customize their version of IDOC_INBOUND_ASYNCHRONOUS (ex. MYNEWRFC_IDOC_INBOUND_ASYNCHRONOUS) to return a response after the status 50 step is complete in SAP, close the connection then, and not wait while further processing continues.
The problem with the IDOC_INBOUND_ASYNCHRONOUS RFC always being used on the SAP ALE call, is that SAP does not reply right away after receiving the idoc from SI. SAP gets the idoc (status 50), then does some processing and moves it to a 64 status, does some more processing and puts it in a 62 status, and finally replies to SI that it got the idoc. In some cases, this takes minutes, instead of milliseconds. When you do this with millions of idocs, there is a lot of wasted time. By calling another RFC for this SAP ALE connection, we can get the response from SAP, as soon as the idoc is received, and move on. The connection won't stay open, and there won't be a delay in SI.