In terms of communication and logic protocols, I don’t see any reason why this shouldn’t work. However, there is a concern with the logic voltage levels. The Alpha Clock Five uses 5 V logic, and the RPi uses 3.3 V logic. Signals coming from the RPi to the A5 should work fine (the A5 can understand 3.3 V logic inputs), but signals from the A5 to the RPi will be at 5 V, and could damage the RPi input pin if it that pin is connected. It is possible to add a logic level converter (in a few different ways) if you need to get data back. Or perhaps, since it’s primarily a display device, you won’t mind not getting data back.
If you have the blue or white model of the A5, it comes with a 9 V power supply, and a (small) voltage regulator to step that down to 5 V to drive the on-board logic. (The red model comes with a 5 V power supply.) The 9 V power supply does have excess capacity, so you can use an additional voltage regulator to drop that down to 5 V to power the RPi. I would not recommend using the on-board regulator to power the RPi; it does not have much excess capacity.
