According to that answer it's not possible, which is a major flaw in the system IMHO. You have a registry and storage, why can't that registry be tweaked at any point?
Unfortunately, I am behind on this project so I am reticent to abandon metabox for another solution at this juncture. I may have to write a custom plugin with it's own form handing to make this one small bit work and that chafes.
