I don't really see any way around it, but maybe some other bright minds around here can come up with a solution.

But the bottom line is we can easily have complex jobs that take considerable time to solve, and each submission would need to be validated.  For example, if I create a job that takes 0.1 sec to solve, and I dump 1000 submissions onto a SN, that node would be tied up for 2 minutes before it could do anything else.  And this will take place while people are dumping hundreds if not thousands of legitimate POW submissions on the SN.

I still don't know of a good solution...I still wish we didn't have POW in xel and just focused on bounties.  I know you'll say we need the POW logic ;-)  I'm just not convinced, and I feel like it just complicates the validation / targetting / volume issues.