Ack, you are correct mark.
I tried to map UP on one stick to the top half of a d-pad gamepad
(up or up-left or up-right)
But Assault didnt want to move when hitting the diagonals.
I think its a conflict between the original game not allowing diags, and mames setup.
I then mapped three keystrokes to "up", and pressed them in all combination's, and there was no conflict whatsoever.
Next I tried a mini analog. Same problem. Turned it to digital mode... which switched the mini-analog into digital "Hats", and all worked fine.
So, this is mostly an issue with mames input system.
If you can map keystrokes to your joys using an encoder.. I think it will be ok. But you will want to make sure you make the top and bottom of the stick for up and down. This will prevent the dead-spot when trying to turn the tank with both sticks ...when you hit a diagonal on accident with one stick.
So, if we used keys z,x,c... the mapping would look like this for "Up"
Z or X or C or Z+X or X+C
The only issue might be difficulty in launching bombs and rolling the tanks to the left & right. However, I think its easier to find exact horizontal with both sticks, so may be ok. Also, you do those moves 10x less than the standard driving turns.