As I recall the prototype board, the hardware buttons on the top row don't span the entire row, so that means you can move the support hole even closer to the corner potentially, it'd seem to me. But without official confirmation of whether such an idea is feasible, it's just another what if and doesn't really get us anywhere.
Put a button in the middle of the speaker then? Mmm.. no.
Something to ponder. Look closer at the arrangement of the letters.
C Y
A B
Z X
If you visualize your thumb coming in from the right at an angle of 135 degrees, it will naturally 'fall' in a line going between X, B then between A, Y to C. Minimal to no movement of your thumb is then required to touch any of these 5 buttons. You will still have to move your thumb down/left to hit Z.