Found a nice workaround: JoyToKey. It allows game controllers to emulate keyboard input (or mouse), so it was easy to assign the keys Blastaway uses to my joysticks. It recognised all my 3 USB devices. Well worth the $7 asking price.
Alternatives: Xpadder (couldn't find a test version so I didn't try it) and InputMapper (seemed a bit complicated so I didn't try it).
So... I can now finally start playing Blastaway!