This page tests whether accesskey is implemented by browsers even when elements are hidden. There are three hidden buttons on the page, each hidden using a different method, and each with the indicated accesskey:
My hypothesis is that Button Z would work, but neither of the others would. As indicated below, IE8 is the only browser that confirms my hypothesis. All other browsers support accesskeys on hidden elements regardless of how they're hidden, except Firefox which supports accesskeys on all elements unless they're hidden with display:none.
If you want to enable accesskeys on hidden elements, and don't want any users (including screen reader users) to have direct access to those elements, use visilbity:hidden.
If you want to enable accesskeys on elements that are visibly hidden, but you want those elements to be accessible to screen reader users, position the elements off-screen.