There exists now a freely available package "Shortcuts`", which does not touch the installation directory.

See also this post.

I can't answer your specific question. However, if you're on a Windows platform, I can offer an alternative way to have shortcuts assigned to custom keys. I use a programmable gaming keyboard. If you are already familiar with these devices, then this won't be anything new to you and you can ignore the following.

Again, this may or may not apply to Apple OSx or Linux platforms. I know it applies to Windows platforms. I won't go into a lot of detail. You can get that by visiting the vendor websites. These keyboards allow you to assign virtually any key sequence to any key. They have a limited set of "extra" keys that can be programmed. Most of these keyboards can also be configured to automatically detect the current active application (eg, Mathematica) and configure your custom key sequences for that application. This means that you don't have to change any of the application's default key sequences. I have several different applications that I've configured with custom keys. The downside is you will have to purchase a new keyboard. But, you can get a fairly nice keyboard for $50-$100. IMO, they are well worth the cost. There are also programmable mice that can do the same thing (in fact, I have a programmable gaming mouse that I use for Mathematica and other apps, too). I personally use a Steelseries Merc Stealth keyboard and a Razer Naga Hex mouse. Razer also offers very good programmable keyboards. I have used a Logitech mouse, but I found their software seems a little unstable from version to version.