Crazy idea: point-and-click for PDF end-users?

[Arvid Grøtting]

Point-and-click is great for those corrections I do myself to a
piece, but quite often someone else will notice a typo only after I've
posted a PDF for download.

Does anyone know of a way to hack point-and-click to do some way of
point-and-annotate?

What I'm thinking of is a function that if you click somewhere in a
PDF, a red circle is overlaid around the clicked object and the
clicker can write some kind of "comment in the margin" about what is
wrong in the score.

(Typically "This is wrong!  It should be an F!")

Sound feasible?  Can anyone tell me how deep I'd have to dig to hack
this?  (Probably I'd need to do some scheme, but would I have to touch
C++ as well?)

Not that I'm saying I'd do it; I'm just wondering out loud...

-- 

Arvid