The AirPort Express used to support a KeySpan remote control, which could be used with an application like a PowerPoint or KeyNote presentation to start, stop, advance frames, etc.
No mention of that is made for the current versions of the Express, so if the current Express supports this, it is a hidden feature.
I don't have a KeySpan remote control here to test. Maybe another user can help with that information.
Unless we hear otherwise, you should assume that the USB port on the Express is for use with a compatible printer.