I'm migrating a legacy PHP project (pre-OO) to Symfony2. On every request I have to:
Calling a specific method in every Controller-Action would really be a bad solution. Your solution to use a subscriber isn't perfect either.
Without knowing your use-case in detail it´s hard to find a suitable way.
Maybe one approach would be to write a Twig-Extension and injecting a Service into this Extension. The Service would get the Request-Stack via Dependency-Injection and compute the relevant Data. You could then access this data "on demand" through the Twig-Extension during the rendering.
Another approach could be using sub-requests during the rendering (Symfony: How to handle common request scope data)
Maybe these tips already helped you a bit. Otherwise let me know more details, where / how you need the data during the rendering.