I know elm is somewhat rot resistant not sure about tannion content.
Elm has an intertwined grain which keeps it together a long time. Ive been told it last a long time when completly submerged. If you have a lot of them on property. You could try one in a sample area. In my area they die out at a young age. They stay standing for what seems like forever. And i know when i dig them out they have a very deep rot system. So if you use the stump this could make good cover. Attach a rope or cable to a big elm stump and see what happens you can always pull it out.
