I want to mix point&click scenes with arcade/minigame scenes. For example, adventure gameplay could change to driving gameplay at some point, like in the highway section of Sam and Max Hit the Road.

I guess I can make a separate "gameplay room" in PowerQuest editor, populate it with my own gameobjects (rigidbodies and stuff), then hide ingame interface and characters after switching to this scene. Will it be the "right way" to do an arcade section?

Please point me at right direction.

Yes, you've got the right idea. You can just ignore the powerquest stuff for that room. 

There's a tickbox in room data to hide the player character, and you can hide the guis in the room's OnEnter function. Eg. G.Inventory.Hide(); 

Let me know if more questions come up as you go.