Altering field passed as parameter to plpgsql trigger

I am trying to make a function that can be reused as a trigger on
various tables but am somehow not finding the correct syntax for using a
parameter to the function to identify the column to be altered by the
trigger. Stripped to basics:

create or replace function foo_trigger()
returns trigger
as '
declare
  ...
begin
  trigger_table = TG_ARGV[0];
  field_to_alter = TG_ARGV[1];
  ...
  new.field_to_alter = some_computed_value;  <<==========
  return new;
end;'
language 'plpgsql';

create trigger foo_change before insert on foo
  for each row execute procedure foo_trigger('foo','somealteredfield');

What is the correct syntax for the line:
new.field_to_alter = some_computed_value;

Also, does plpgsql have a preferred way to identify the table that fired
the trigger. I see the syntax for C but not for plpgsql.

Cheers,
Steve