Any idea what this hole is/was for? (obviously water, but to where might it have gone.....heater core)? 

Yep, heater core. 

    Lastly, to plug this hole, should I attempt to tap it with a NPT tap, or is there something that is a press fit that I should purchase to plug this hole?    

Normally, there's a steel hose barb that's press fit into the housing.  I just looked at the Mazda parts list, and didn't see a part number for it.  I don't know if you could find an appropriate sized pipe to drive into the hole, and if you could trust it if you did.  I've always wondered if those pres-fit items could be replaced, particularly since my (formerly) junk engine's heater pipe is rusted severely. 

If you don't plan to use it for the heater, then you might be able to find an appropriate sized freeze plug.  They also make the removable rubber freeze plugs, that you expand by tightening a bolt.  That might not give you the appropriate warm fuzzy feeling in a plane though.  If it happens to be close to the right size for an NPT tap, that would work too.  I think you have some room to stuff something in there to catch the chips.  

Rusty  (lots of typing, but not much help)