And that's why I'm paying particular attention to the FaceBook standard, because I expect that will become more well known.
Hmm... hardly a standard, though. You'd be better off looking at some government/federal sites. They are more likely to implement a known accesskey standard. The UK government certainly does and most UK-based disabled users will be familiar with that system.
Does anyone know, BTW, whether / how accesskeys are implemented on mobile devices (if at all)? I take it they're only available on devices with hard keyboards?
If you use the accesskey attribute, you should be able to hook straight into the browsers shortcut handling - irrespective of whether it's a mobile device or not. Pretty sure that Opera Mobile will support accesskeys but I've not tested it for ages.
I'm thinking of putting some Text in the footer to make people aware (perhaps links labeled '0'..'9' with alt text displaying what each number does)
Nice try but that won't work or sighted keyboard users. They don't have access to alt text - yet they far outnumber non-sighted users (who have their own software shortcuts) and are likely to be the group that rely on accesskeys the most. The info must be perceivable and understandable. The best way around it is to have a footer link to a page that fully outlines the access keys available. Even semi-experienced keyboard users will look for such a link (especially in the footer). Kill 2 birds with 12 stone and make it a full-blown access policy/support page - eg http://quirm.net/accessibility-statement/.
If WP folks could simply agree on something and then implement it, then it would probably become a de facto standard overnight
I'd actually argue strongly against WP adding any access keys. Although the concept of access keys is sound, in reality, they often cause more problems than they solve because they conflict with, and over-ride, the user's own software. The issues are magnified greatly when you have a multi-lingual audience. The last time I and a group of others researched the issue informally, we found that there were virtually no keyboard combinations that wouldn't create a problem for someone.
My advice - leave access keys alone. Focus on creating an accessible site instead so that pages can be parsed by the largest number of user agents. Then let the user's own software and the user control what shortcuts they do, or don't have. They are far better placed to address their own needs than you are by imposing a global system.