Yeah, I'm thinking forward would work better. You'd need a shaft supported by a bearing in the middle - attach the pulley to the left side and the crank arm to the right. Mount the whole thing high enough that you don't hit your knuckles on the base and with the pulley in-line with the sewing machine's pulley track - some arrangement to fine tune the position would be important. The mount I'm envisioning would look something like a fixture used to mount a handrail to a wall, though it probably wouldn't need to stick out that far in front. The mounting bracket would need to be fairly sturdy and to attach to the base very securely since it will need to resist the tension of the belt and the moment the user would impart if they're not moving their hand in perfect circles.