Detecting which columns a query will modify in a function called by a trigger

I need to implement a fairly fine grained security model. Probably a bit
finer that I can do with the standard ownership functionality.

My thinking on this is to create a table that contains the users, and a
"permission bit" for each function that they may want to do, vis a vi
altering an existing row,or rows, or inserting new rows.

Looks relatively straight forward, if fairly time consuming to do. But I
would need to know which column(s) a given query would add..alter from the
function to implement this via a trigger. looks like I see most of what I
need t do this in the docs, but I can't quite figure out if I can get this
down to what column(s) a given trigger will modify. Is this possible?

--
"They that would give up essential liberty for temporary safety deserve
neither liberty nor safety."
-- Benjamin Franklin