You are correct Dave. You cannot push this particular butt plate forward very far without opening up gaps behind. The lug on the forward end of the buttplate return acts as a drawer to pull the extension down and as a stop to prevent it from going forward. The heel screw simply anchors the plate against the stock.
The cross pin idea could do the same work. Once the return is fully inlet, it could be clamped down hard into its inlet and then cross drilled for the pin, assuming the heel is also bottomed out against wood. Then the lower screw could be set up to pull the plate down slightly, further tightening the plate's inlet.