Have you considered goats? (I have no idea if they'll munch on sea buckthorn, but it might be something to consider.)
We're investigating grazing sheep on ~4 acres (~1.5 hectare) where wild blackberries are a problem and have been told that they will help keep it under control.
If not goats, other livestock may work. (Pigs, from what I understand, may be of some assistance with digging up the roots. Just don't let them get loose into your reserve, because then you've got a problem with pig control instead of weed control...)
You said in a comment:
In this instance it wouldn't work for the very reason you state - there is no separation between the buckthorn and the reserve.
(Again, I have no direct experience, though a friend of mine keeps goats and I've done research to see what would work for us.) Depending on the type of goat you may be able to keep them in an area with portable electric fencing and a small portable fence charger.