Cliff, as best I can tell; you are on the right track with the BM18005 Rear Aux. Hydraulic Kit.
BM18005 Rear Aux. Hydraulic Kit
By the diagram view, it appears to plumb into some ports in the vicinity of the rock shaft valve cover front side by the rock shaft drop control valve.
Do you have already have the BM18005 Rear Aux. Hydraulic Kit?
I suspect you already realize this but it appears that you must have a selective control valve part number AM117842 at $765 to be able to plumb the BM18005 Rear Aux. Hydraulic Kit into the system as it definitely appears that you it plumbs into the SCV. In a nutshell................. this sucks! I called my buddy Ricky at my Deere dealer, he is the parts manager and he put in touch with their CUT lead mechanic (Fuzz) (don't ask me how he got that name

) . Best case is that as long as you have an SCV on the implement you will be using you can use the BM18005 Rear Aux. Hydraulic Kit................... BUT................you will have to locate that parts for or fabricate some type of hydraulics fittings assembly to go in place of the SCV. You may be able to run your 790 by a hydraulics shop and have them take a look. They may be able to fabricate what you want much cheaper than buying the Deere parts kits and fabricating something to use the kits.
Sorry I could not be of more help Cliff. If you would like to talk to the guys at my Deere dealer, the toll free number is1-800-726-7172. Ricky is top knotch when it comes to parts and offers a discount to TF members and referals. Tell him Randy referred you. Many times they can find a part and have it UPS'd to you direct. Your dealer should be able to do this as well.
Anyhow, good luck with your 790 hydraulics power beyond modification. Be sure to let us know what you come up with to get this worked out and post some pictures. And don't be a stranger.