How about using the subsite_id instead? This is not going to change for your custom site, even if you rename your subsite. I would just have one additional keyword like "custom" which is applicable to all pages regardless of subsite.
Having an "off" switch will most certainly help. Caching does not make any sense in my opinion. But to speed things up I would use a delimiter not commonly used in URLs, like "index+custom.tcl", so the RP can very quickly check if there is a file that matches this pattern in the file system (if we were to use "-" this will bring issues with index-2 and so on and so forth)
