Its a good question. And its actually one of the big reasons that they urge you to synch ALL data to SFDC so you have that campaign history.
Even if its that you're synching the lead to a hidden queue and reassigning once it hits that lead score.
There could potentially be a workflow by creating a validation rule on a lead score threshold so that you can't save leads in SFDCunless they have a certain lead score...then you to add a member to a program (that is synched to SFDC) in Marketo, and then once they hit the score and pass the validation it would add the individual to all of those programs.
Problem with that? It would add them all on the same day...which isn't ideal. Also it would prevent any leads from being manually entered in ways other than Marketo.
So long story short, I'd still push back to synching everything so you can collect the campaign data (and so you can report on it in SFDC, which is the bigger plus). Then just expose those leads to sales only when they hit a certain score.
If you don't want to do a queue...I've also seen it that they don't move to "Open" status until they hit a certain score. That way they wouldn't show up in the Open Leads view for the reps.
Hope that helps!