Actually, in friends matches you can set it to stock, it's only the random matches that you can't.

All you do is set your normal Multiplayer matches to STock, and then enter Wifi and create a room.

The default will be set to the same as on your Wii, and you can alter the number of Stock on the Wifi connection during Character Selection as you could normally.

I played a few stock rounds on the Wifi already.