Can an opponent respond to Bryn Thundersong's 'When you play me, opponents can't play cards this turn' trigger with a reaction speed card like Zhonya's Hourglass?

Ruling: Yes, opponents can respond to Bryn Thundersong's trigger with reaction speed cards like Zhonya's Hourglass because it is a 'When' trigger that uses the stack.

Sequence:

• Bryn Thundersong is played
• Bryn's ETB trigger goes on the stack
• Opponent can respond with reaction speed cards before the trigger resolves
• Once the trigger resolves, opponents can't play cards for the rest of the turn

Nuances:

• The key distinction is that Bryn uses 'When' language, making it a trigger rather than a replacement effect
• ETB (enters the battlefield) triggers can be responded to at reaction speed before they resolve