Yep, pH adjustment of tap water by The Man is in the range of 9.2 to 9.6, very intentionally, using primarily sodium carbonate and fine adjustments with CO2.
Sulfur deficiency was my other guess: my sources (cough google) put Fe and S as my two best guesses, as they impact new growth and turn the leaves fun-and-incorrect colors. I leaned towards iron because yee olde soil nutrient uptake chart suggested that iron is still available at low pH, and I was trying to keep both nutrients and soil in the 6.5-7.0 range. Sulfur seems to be most available in pH 5.5 and up, and I feel like I’d have many more problems if things were that low. Knock on wood.
Per this thread about a maybe-probably potassium deficiency in another plant (still ongoing), @ryasco and @lotus710 expressed fear about my tap water when I told them the pH was above 9, so I picked up an RO/DI filter. I’ve been using a mix of RO/DI and non-deionized RO (which has a pH strikingly close to tap water). If I mix up a full strength batch of General Organics nutrients (the whole kit), with tap water, bottled water, or RO/RO+DI water, my pH ends up in 4.5 to 5. Tested with a calibrated pen, and verified with universal indicator pH test strips (good lab ones, not crappy hydro store test strip).
As said, I gave some kelp+alfalfa compost tea, with a dash of bottle nutrients and epsom salt, and new growth seems to have greened up a bit. Oh, also, a thin layer of lobster compost mixed into the top (only non-garbage I could find). It is growing, and the new growth looks a bit better – if I can keep new growth going, I have time to fix the rest 
I’ll look into a friendly foliar feed, hopefully be able to try that tomorrow, and post some pics of the current state of things.