In brief I'm suggesting:
An option to enable php-fpm by touching /srv/domain.com/config/fpm
I'd like this to create the necessary apache2 config (seems generic enough) and perhaps merge config lines from /srv/domain.com/config/fpm into a custom php.ini for each pool (one per domain?).
I suspect this can be done without getting in the way of anyone doing a custom fpm setup for a site symbiosis isn't managing (for whatever reason they might have to do that).