The interesting thing is that on the Integration tab of PO's Company Profile, we have:

Send to AP reference field: Original doc number
Send to AP description field: Receipt number

Actually what it sends to AP is the RC (Receipt number) to the Reference field and the ADJ (Invoice) to the description field. Document Code it's not even an option that's available but we're getting the ADJ# in AP. If this is a glitch, it is a great one!

Hope you can add the ability to send ADJ#'s to Ledger.