Digital BW, The Print

Why does a system need to apply glop in registration over specific
tones/colors/whites? 

...why not apply it over the entire rectangle, like spot gloss in
lithography?  

If that would work it'd be much more elegant (scientific elegance =
simple, reliable, effective).

Why can't glop be put in a spray can...wouldn't it be better than the
various clear urethanes (which don't adequately deal with bronzing)?  

Glop, applied like a pigment or ink, sounds like a very temporary
solution, even assuming it proves to be workable (like "open loop
systems" and "profiles" and dye transfer and daguerrotype).