So, first of all, I would recommend checking the event outline and such about this. It’s possible that FFG is pickier about Nationals being run with whatever’s official at that moment than one might expect. Also, I assume that you have a good enough way to communicate any changes to players that you aren’t worried about someone showing up with pre-rotation decks because they checked the FFG legality info and not your site.
That said, it doesn’t look good for the MWL coming out in time for you.
On the other hand, I’m not sure FFG has said that the FAQ will be released on the first. They’ve said it’ll take effect on the first, but that doesn’t rule out the possibility that it’ll be released before then. So there might be hope, but nothing I would count on.
If you want to try to get more info about the MWL release date, tweeting at FFGOP is probably going to go better than trying to contact Boggs.
My advice would probably be: pick a date. Announce to players that if the MWL is released on or by that date, you will use it, even if it hasn’t officially come into effect by the time of the tournament, and that if the MWL is not released on or by that date, you won’t use it (and perhaps won’t use the rotated card pool), even if the MWL comes out after that date but before the tournament.