I faced something similar today and I would like to add my findings to this discussion.
First of all, I’m using nginx as a reverse proxy and apache with multi-php. I need to have some special php settings at one of the sites, so I made a copy of the PHP-5_6.tpl and named it PHP-5_6-OC.tpl. But that didn’t work. Then I tried to name it PHP-5_6_OC.tpl but that didn’t work either. Then I opened up the file func/domain.sh to check the code.
If I’m not mistaken, the following code checks the template filename to decide on the php version to use: $backend_template =~ ^.*PHP-([0-9])\_([0-9])$
So, in order for the template to work, the template filename must be named like whateverPHP-x_x.tpl (where x_x the php version).