Can helicopters perform the Split S maneuver?
Yes, but you need a rigid head.

A rigid head does not have any hinges, instead, the blades flex up and down.  

Wings don't know or care if they are upside down or right way up, indeed, most helicopter blades are symmetrical.  It only cares how much airflow there is and at what angle it hits it.

Here's an EC-120 performing a split S and a Lynx showing what's possible. 

Perhaps the best around is Chuck Aaron, displaying for Red Bull.

Here, Dennis Kenyon shows about as good as it can get with a semi-rigid head.  He gets close to inverted but never actually flips over.