I can see no technical reason why Apple couldn't make an adapter for the G4 towers to use the airport extreme card. The only problem I could see would be if there are major antenna requirement differences between the two cards.
On the other hand it could be that the MDD G4s are an interim model and a new one will be released shortly with DDR RAM, FireWire 800 and Airport Extreme capability. In this case Apple would not make an adapter to leave Airport Extreme as another factor to upgrade to a new G4.