This is all I have, but it is nearly useless. It appears to attach to the trigger guard front. The muzzle end would also certainly have to attach to a stirrup sling attachment on the foreend, presumably through a barrel lug?
I would contact James Rogers. He studies lots of old paintings and engravings, and may have advice for you. He can also make the associated period metal work. I would use James as the "one stop shop" for a Jaeger sling.
Ed Wenger has also studied the early Jaegers, and may have some advice.
Hope this helps. God Bless, Marc