You can close off the back of the shower with rigid foam board insulation, but you need to properly detail that cavity with regards to the rest of the house's thermal envelope.

ie, if you simply fasten RFBI to the back of the shower, you've protected the plumbing from cold air from behind. But what about the top? Or the floor, through the joist bay?

I wouldn't encase the plumbing behind the shower wall in spray foam. I'd prefer that warm air be allowed to pass through the fiberglass wall, warm the plumbing cavity, and keep the pipes warm that way. By properly detailing the back cavity (back, top, bottom, sides, etc) that encloses the plumbing, the plumbing cavity will essentially be a part of the heated part of the house.

I hope that makes sense.

Thanks for the input. Bottom/top/sides of the shower are all enclosed as part of the original construction. Just the back side (opposite wall from showerhead) is exposed to the attic. I was thinking along the same lines as you, rigid foam board across the back and caulk it to seal out the cold air and go from there.