So, rather than leave this hanging I'll answer it.
What I was asking may not be technically impossible, maybe via @Chausser's proposed nesting detection technique, but it's at least difficult/unwieldy.
In the end I solved the problem by simply using Symfony's normal customised Form rendering features - adding each field etc into exactly the right place within a detailed Twig template. I was completely new to Symfony 2 at the time of asking, and didn't even realise this was possible!