Sending one of the minecart variants added by Mo' Minecarts through a cart assembler successfully assembles the contraption (which I hope is intentional behaviour, the piston cart interaction is really cool)

Picking up the minecart contraption with the wrench afterwards, though, and placing it back down, will replace the minecart variant with a vanilla minecart.

This allows for an exploit to let you create iron for very cheap by converting wooden pushcarts into minecarts and melting them down into molten iron.