There is a body shop around here that has said they have put a SPEED3 engine and tranny in a Mazda5 they also did other stuff to the body... and they had to make the hood... anyways I've never seen it.
If you search around there've been several discussions on this topic, but given the state of the economy as it is today the chances of making one are much slimmer IMO. Mazda will focus on its Mazda3 and Mazda6 initiatives globally instead I'm sure...