I researched a lot on this and seem to be getting conflicting answers on SO and all of the web. I understand that with Section 508 that compliance DOES NOT equal accessibility
The problem with some standards (as well as many laws) are that they're open to interpretation...
The only mention I can find in the 508 standards that mentions keyboard use is this (verbatim):
Subpart B -- Technical Standards
§ 1194.21 Software applications and operating systems.
(a) When software is designed to run on a system that has a keyboard, product functions shall be executable from a keyboard where the function itself or the result of performing a function can be discerned textually.
My spin on this is:
Another end of the spectrum is the WCAG which is pretty much coupled with 508 compliance, and in my book better defined: Keyboard stuff is under 'operable' in WCAG.
In a nutshell: It's good practice for UX to have custom keyboard shortcuts for important features. But has no bearing on 508 compliance by itself. (With exception that functionality should be reachable by keyboard -somehow-).