Matt,
I just looked at the service manual online.. And you are right! It does blow into the engine side.. Opps!
But I still think its necessary to hook up the vac lines to the intake.. And here is the reason why..
I think the reason why mazda did this is to reduce the oil consumption when the engine does not need as much oil, ie idle. During idle, there is almost no vacuum created and air is being sucked thru the nipple of the jets.. As engine goes into load, the vacuum created by intake, blocks air going into the nipple and cause more oil to be injected into the engine. If you just put a filter there, air will still be sucked in vs. oil (but not sure what extend oil vs air will be injected into the engine). Well, that's the new theory I have.. I should have double check the flow of the jets instead of guessing
Also, you are right.. I also forgot that after intake ports on the iron is exposed, the oil meter jet opening is exposed at the same time. At that point, it will see pressure and hence 1 way valve must be one direction.. Sometimes I'm very stupid