Since no one responded to my question about how to use a keyboard shortcut for autofilling card information, I assume this functionality is not available.
Since the Ctrl-Shift-L keyboard shortcut works well for autofilling login information, why not also implement a hotkey for triggering autofill of card information (and perhaps also a different hotkey for autofilling identities)? As with the login hotkey, repeated use of the key would cycle through the available cards (or perhaps some additional features could be implemented to facilitate autofill when a large number of cards are available).
Autofill on page load should also be considered. Perhaps a list of URIs for which autofill would be triggered (and/or prevented) could be part of the configuration of each card item.