TotalTerminal issues a warning that the F7 key cannot be used as the hotkey because it is already being used as a system-wide hotkey. However, I’ve made sure that F7 is not bound system-wide AND I am able to bind it in TotalFinder’s visor just fine (as a test). This is on Yosemite 10.10.
I’d recommend taking a look at my last post, linked below. It will take some work for you to do the same, but it’s probably all that will work. I also had to use Karabiner recently to get F4 to work right; you might be able to use it as a work-around that is simpler than hacking plists.