Certainly should be tested on a range of OS versions as to whether it interferes with the OS' normal operation for installing (all manner of installing), managing and even previewing fonts.
Even if there are no issues, I still cannot see why to invest the time in such a feature. It's not onerous to use the menu system.
Most programs have an option to open supported file types from Windows Explorer, and I don't see why FontCreator should be any different. I can set *.ttf (though not *.otf) to open in IrfanView by default or from the Open With menu so why not FontCreator?
It is optional, after all, though maybe fonts should not be associated by default on installing FontCreator as that could mess with users' preference for previewing fonts in FontView or other programs.