But we can assume that, once Metol is in solution, its free base ions float around freely, yet they don't precipitate when more Sulfite is added.

Could it be that Metol dissolves better when its free base part can fully ionize? In other words it may not so much be an issue with Sulfite, but more with Sulfite's high pH. It may equally hard to dissolve Metol in Carbonate. The reason that the issue is usually associated with Sulfite may come from the fact that unlike other alkalis Sulfite is needed to preserve the Metol.

When I get to it, I'll try to dissolve some Metol in 30 g/l Sodium Metabisulfite and in 30 g/l Sodium Carbonate. My guess is it will dissolve in the Metabisulfite but not in the Carbonate.