I emailed Curt about this (pretty sure the Even-speed method was his). I don't think he'd mind me reposting the exchange:
Me: Mr. Stock,
I came across your Even-Speed-Mead technique on the AHA forum. I was
wondering if you had tested using calcium hydroxide instead of
potassium hydroxide. If so, was there some sort of negative flavor
from the calcium? What do you think the threshold for too much calcium
Calcium would help with pH adjustment, but as a vitamin for the yeast, potassium is better choice. 300ppm of potassium is ideal.
Not sure if there would be a flavor issue or not. Good question though.
I have used calcium carbonate for pH adjustment. Too much carbonate can affect the flavor and make an environment that accentuates harsher compounds.
When adding any thing to meads I like to keep the amount as low as possible. Unless it's fruit or honey!