"I don't see why that wouldn't work." because of the threshold. But I can see that in fact the GUI don't set a scrub with an minimum interval but rather execute a scrub regularly and then the scrub checks if it executes or not. So a custom CRON should work. Thanks, I'll try that ;)
Edit: CRON added, there is only to wait now. BTW putting 9999 in the threshold field works... :D
There's no threshold when you run a scrub with 'zpool scrub' is runs it now, no questions asked. So as I said, I don't see why that won't work.
You can change the threshold on the built in one if you want, but it's not required. As I said, if you're forcing a scrub every two weeks through cron, then the 35 day threshold will never be reached, so a threshold of 35 days is exactly the same as one of 9999 days.