I'm not sure this qualifies as an 'answer', but being completely stumped and seeing there were several similar unanswered questions about scopes and Tire on SO, I would recommend anyone else having the same issues to simply use another search client.
My criteria were that it had a free tier production support on Heroku, was relatively easy to implement with good documentation, and supported my existing scopes.
I have gone with Swiftype, which meets all those criteria, providing excellent documentation, a sample app, and a free tier on heroku.