There seems to be a fix now in Tk for the key event issues on macOS. The caveat is that we will probably need to ship with Tk 8.6+ as this fix will not be back ported to Tk 8.5.
One issue could be whether Tcl/TK 8.6 can be built on a macOS 10.6 system. I'm not 100% sure. One option would be to have binaries for 10.6-10.11 which use TK 8.4 and binaries for 10.12+ which ship with TK 8.6. It could be a pain but the scripting already handles most of the headaches.