Nima. I like the per subsite customization. That works for me.
I think your idea is much simpler and will work well. ie: page-name-subsite.tcl could be the default. Any reason to not use the URL of the subsite and forget using parameters?
I can see turning OFF this feature to speed up the request processor, but I don't think it makes sense to bother making the suffix configurable.
You'd need access to the filesystem to create the custom file and there's little chance someone will create a file that matches the pattern accidentally.