Please see the most recent updates in the "Where did the .com name go?" thread. Posts number 16 and 17.
The A2A P-51D and P-51H both have dropping tanks as stock items. Use either the VC switches or the ones on the pop-up panels to activate the drop command. If I remember correctly they are marked as 'Chemical Release' on the switches. You have to click them twice, once to 'arm' them and the second to drop them.

but was wondering if you can make a gauge that can do that automatically once you loaded the plane/panel.