Viewser: Because there is no word for the participant who is used in interactive art.
Viewser: Because 'User' sounds like drugs, telemarketing, and microsoft excel.
Viewser: Because Viewers can't touch or move the art.

viewse : [Pron: vee-yooz] –verb (used with object)
1. To see and interact with at the same time. To experience.

Variants:
Vuser: more user than viewer.
Vuying: viewer/user/buyer | ex: I vought a Warhol Brillo Box.
Vooser: a drunk viewser | ex: that dude just voosed all over the place.
Vuice: a genre of art that requires drinking juice to succesfully experience the piece.

Addendum: I didn't make it up. ; (