I'm an avid saison brewer, and I think this recipe looks really interesting. Here's my thoughts:
You're gonna get some lemon/lime character from the Motueka hops. So if you want it to be really subtle on these flavors, you could potentially drop the zest and just use Motueka as both your flavor hop and for a dry hop. If you want something more on this spectrum, I would go up to 1/8 oz. zest total for 5 gallons. So maybe 1 or 2 grams of each zest? Also, you'll want to add the zest close to flameout I would imagine, to preserve as many aromatic oils as possible.
I love the idea to use WLP 644 to ferment. Will have to try that myself. Because it's Brett, it's gonna eat a wider range of fermentables than normal Saccharomyces yeast, so I wouldn't be afraid to mash a little warmer, particularly if you lose more than 2 or 3 degrees over the course of your normal 90 min mash. I've started saison mashes anywhere from 143 to 154, and the more typical saison yeasts are so highly attenuative they've always gotten it down below 1.008 FG (the 143 mash even went down to 1.000 FG).
Finally, you might want to use some rice hulls in the mash with the combination of rye and wheat (the rye especially), or maybe just go down to 5-10% of the grain bill on the rye. I've definitely overdone it with rye in my rye IPAs before, and you might find that it's character overwhelms the more delicate citrus character you're trying to get.
You'll have to let me know how this one turns out... looks like a great idea on paper. Cheers!
EDIT: One more thing I just realized... Saaz seems like a strange choice for bitterring since you'll lose all it's aroma over a long boil and since it's so low in AAs. Since you're going for some citrus notes, what about something like Galaxy. Nice and high in AAs, and absolutely loaded with citrusy hop oils. Granted, the vast majority will boil off during bittering, but you could try something like 1 oz. at 30 min for all your bittering needs and still retain some oils.
