I was wondering, in a big walk-in closet setup, do I need a ventilation setup for an HPS system? And if I do, are there any recommendations on how to do it?
It depends, but most likely yes. That lamp will give off heat that has to be removed. And the plants will need fresh air anyway. Any number of ways to do it, but you have to tell us more about your setup. Can you access an attic or crawlspace through the ceiling or floor? You could put a door on the closet with intake and exhaust holes cut in it.
Of course you do but as tasty said, tell us your whole setup and limitations etc. the more we know the more we can help.