Back in the UK for a few days now. I managed to get the asynchronous pathfinding added to the UESVON plugin yesterday.
The code is super rough right now, as I just copy pasta'd the existing AI MoveTo task and chopped it up to suit my needs.
On the plus side, I kinda know my way around the behaviour node boilerplate in UE at last. I'll give the code a proper cleanup after Christmas and start thinking about properly polishing it up and making it API stable.
Still some infuriating edge cases where pathfinding seems a bit off sadly, but at least it's not routing through obstacles anymore :)
Have a good Christmas!