That's a tough question. For the next version of LocomotiveCMS (v3), we'll use a full stack of middleware so that it will be easy to add your own custom behaviors.
But, for the current version of the engine, it's not that easy. However, I'm thinking about something. Perhaps you could create a Route constraint for the mobile detection (http://guides.rubyonrails.org/routing.html#advanced-constraints).
Then, you would modify your routes.rb by adding this:
match '*path' => 'locomotive/public/pages#show', page_path: 'mobile-index', constraint: MobileContraint.new
Never tested so it might not work :-) Happy patching!
Didier