Combining JAL PY and AA Y on award ticket?
 

I know that some partners like BA will allow this, but as far as I can tell, it's impossible to book such an itinerary with JAL miles... is that correct? Trying to figure out the best way to book DCA-ORD-NRT with the transpac in PY.

(While I'm at it, does anyone know why JAL has four different oneworld award charts for members in different regions... and the prices on each seem to be exactly the same???)

Reality is, JAL has four different systems handling the four user regions represented out there (Japan, Europe, Asia, Americas). They are just keeping things tidy on their side by showing the same charts for each region.
As to your question, the rules allow it (link). You just need to call it in if you cant book it online.

You are linking to partner award redemption which I don't think allows partner + JL redemption anymore. The only reference I found on partner redemption is "One award ticket is valid for travel on one of the following partner airlines" without stating combination with JAL is allowed.

To redeem this as a single ticket, OP will need to use oneworld award, which doesn't have premium economy redemption. And it is clearly stated on the "Conditions for Using Awards" page that "Premium Economy Class is not eligible with an Economy Class award, but is eligible with a First Class award or Business Class award."

Thanks Pak, that's what I thought. The Japanese airlines really do their best to make PY difficult to book.