AFAIK you can only use a keyboard while on the XMB (to perform any task, like X button, O button, etc.), just the button placement isn't as obvious. For X it's Enter, O is Escape, Square I can't remember and /\ is F1... Arrows are D-Pad/Stick... And then the shoulders, can't remember too.

Games have to specifically support the keyboard, the only one I know is UT3 for movements, and a mouse for aiming (it's an FPS.) I this WipEout HD was built with the Dualshock 3 in mind (rumble for incorrect pitch) so I really doubt a keyboard will work. But you can always try if you have one.