You could use Lead Score value, and set up a campaign that says "if lead score is less than 50 then don't sync"
Then have some related triggers, like
> if lead created in APAC region, apply lead score of 60
> if lead score under 50 = do not sync
Would achieve the desired result, you just have to make sure you accommodate all scenarios, like if the lead is created manually in SF it still triggers the lead score Vs it being created via a marketo form and being assigned a lead score, etc etc.