i am using a technique to set some omniauth options into the user-session.
i guess that this might work for your solution as well. you still have use the /auth/:provider
routes though!
so i am basically linking all login urls to my auth_path
# routes.rb
get '/auth/login/:provider', to: 'sessions#auth', as: :auth
once i extract the data and put it into the session, i redirect the user to the right login path. in your case, it would need to extract the data from the params and then redirect:
# sessions_controller.rb
def auth
session[:omniauth_keys] = Usergroup.omniauth_keys(params[:provider], request)
redirect_to "/auth/#{params[:provider]}"
end