I was able to figure this out. That is to say, I was able to make it work. I figured I'd post my solution in case others have similar challenges. If anyone has refactoring tips, I'd love to see them.
firstHandle = "view:'DFSAdjustableTransactionLineView' marked:'Position Leg Handle' index:0"
tap_and_hold(firstHandle)
dest_frame = accessibility_frame(firstHandle)
frankly_map( firstHandle, 'FEX_dragWithInitialDelayToX:y:', dest_frame.center.x-50, dest_frame.center.y )