The only way to work through that as I can tell is to create an action and
assign it a keyboard shortcut.
That means recording your canvas size operations entirely, and accepting
the result. Then in the actions palette, make sure that the dialog box is enabled so that when you press the hotkey, it will display the canvas size dialog box with your previously recorded settings.
Unfortunately, that would require me to make not 1 but 9 separate actions, one for each anchor position. Given the limited number of available keyboard shortcuts for actions, this would be quite impractical. (Even with CS's customizable keyboard shortcuts, there aren't that many more available, a fact that dismayed me when I played around with it on someone else's machine.) It'd be really nice if there were some way to just use the keypad for that, since it's already 9 boxes in a square, just like the anchor setting. (I use the numbers on the top of the keyboard for actually typing in numbers, so there'd be no conflict.) I wonder if there's a suggestion box where one could make such a request for future versions? (Not that I've ever once had anyone take one of my suggestions on any other wishlist I've ever posted to...)