Don't most HPC tasks consume lots of CPU? If you compromise the account of a known userA who runs the known taskB, how will you know that the task isn't doing what it says it's doing: surely you just see the processB, owner userA, at roughly where you expect in the process monitor?
This sort of hack could be very damaging because it's not just compromised accounts - what if they've compromised some of your code? Every time you run it you get bogus results ... and someone else gets cryptocurrency.
EDIT: I know nothing whatsoever about HPC, although I'd love to.