I did this, but it was a hardware solution. I’m using a HHKB Pro2 (any USB keyboard will do) plugged into a BT-500. Not only does this device turn wired keyboards into BlueTooth keyboards, it introduces keyboard mapping & macros.
In my case, mapped the Insert key (Fn + \ on my keyboard) to launch Drafts:
To set this up, I open up a text editor (Drafts works well!) and enter command mode (Ctrl + Option + Insert). Then I type the following to create my macro and map it to Insert:
macro edit 4
0xc000221 delay:500 d r a f t s delay:1000 enter
map add insert macro:4
save
By the way, another nice mapping is Forward-Delete for Lock (holding down Lock triggers Siri):
map add del_fw 0xc000030
save
I sure miss my mechanical keyboard and keyboard mappings when I’m away from my desk!