Setting (menus-deep) keyboard shortcuts...

I’m using Mac OS X…

I know how to set keyboard shortcuts in theory…
Where I get lost is how to set a keyboard shortcut for an item on a flyout menu)

For instance, the services menu under the [application] menu has a list of services, then under each service (group) there is a list of actions…

How would I assign a keyboard shortcut to one of those?
If not available in the std. Keyboard & Mouse Pref panel, is it available in some other context?

In reality, I’m trying to shortcut the copy as PDF item in Mathematica… (Edit>Copy As>PDF).

Any ideas?