so what you want is to add your own comment to a log in which another person sent you Gold?
I guess that makes more sense, however as eerock stated I feel this will just cause headaches from a mediation standpoint trying to track down matching gold logs and the legitimacy of the comments added.
for instance;
if I trade with a user for x amount of ber runes on Monday,
then Tuesday I make a trade with the same user for a griffons, what's stopping that person from just adding the custom comment "griffons" to the prior gold log and claiming they sent gold for the item when in reality they are just relabling a trade from the day before.
now it's cause for additional prerequisite logs to keep track of when things were changed making it harder for site mods to do thier jobs.
just my personal opinion though, good luck!
You can easily avoid all the trouble by making the comment you can add yourself an additional column titled "notes" only for you to add notes yourself