Have you tried incorporating shift-key presses? I mean, hold down the A / B / a trigger button and then all of the other buttons can take on an additional function (not knowing your button map means that could be a ludicrous suggestion...!). But you could apply that philosophy to the keyboard - have shift (or another key) key presses, which reduce the number of keys the player has to reach for (assuming the player is not a pianist). Still requires a lot of learning on the part of the player, though, either way I guess. I don't think there's anything wrong with stipulating gamepad only, but that might frustrate a few potential customers... Not totally helpful, I realise!