I want a two-key macro that will call up the “System Text Preferences” sub-menu of the Auto-Correction subset of Scrivener Preferences. I want to then, immediately, be able to start imputting whatever bastardized shorthand item that occurs to me.
In WORD I’ve long done this whilst (I love sounding British) working on whatever. In Scrivener, at this point, the best I can do, when the inspiration to add or alter a shorthand item strikes me, is to laboriously (o the toil; o the limp-wrist-to-forehead travail) invoke Scrivener Preferences and go through that process).

Firstly, that sub-item is actually the OS X System Preference, so giving it a shortcut probably wouldn’t be too easy.

Second thing: have you tried Scrivener’s built-in auto-completion system? It’s quite a bit easier than mucking about in preferences. Try selecting the word or phrase you want to add to the list, right-clicking on it, and selecting Add Selection to Auto-Complete List. You can maintain this list in Project/Auto-Complete List...