If I play Brynhir Thundersong, can the opponent still reveal hidden cards this turn?

Ruling: No, the opponent cannot reveal hidden cards this turn after you play Brynhir Thundersong. Cards from hidden are also played, so Brynhir's effect prevents revealing them.

Nuances:

• If it was somehow the opponent's turn when you played Brynhir, they can still hide cards (the restriction only applies to playing/revealing cards)