Wondering if anyone has experience using well nuts to attach things to
mast (e.g., lazyjacks) instead of pop-rivets. It seems to me using a
brass screw with a well nut (w/ brass insert) has two advantages - no
dissimiliar metals are touching and its removable.

If you attached a stainless fitting to the mast, then there would be
dissimilar metals there unless you used heatshrink or something on the
screw. Or, if a well-nut with 18-8 stainless steel existed you could
use that.