So I was looking for hours yesturday for this and looking around the best I found was a few people just saying its not locking the value like if they wanted chains at 99 then froze it it would never change the chain.
I tried it with cheat engine myself, they're right it wasn't changing no matter what I tried.

I remembered today for some reason that in the JP version the chain system started 1 kill "after" the pal/us version did. So instead, I searched for a value 1 under what my actual chain was to see if they kept the original chain line of code in the game and just "added" another line of code to start the chain earlier.

Which it turns out they did, which is why no one was able to figure out why, a different line of code is controlling the actual value.

Might be a few years overdue here but better late then never I guess for anyone googling in the future here.

You can try it yourself if you like, just scan 1 number under what your chain number is instead of the exact number of your battle chain.

Just for reference, for USA ff12 the address to "control" the battle chains
is 20578974.
I'm guessing because the "original" jp script code still controls the battle chain, and its just a meaningless line after it in the usa/pal versions alter it only, that the above one might also work in pal and international. At any rate if it doesnt you can scan yourself just use -1 of what your chain is
I'm half asleep typing this, so just to simplify here, if your battle chain is 20, normally your scanning with C.E. for a value of 20 then scan again at chain 22 for 22 etc.
But for this if your at chain 20 in the game you actually scan for 19, then at say game chain 25, you actually scan for 24.
After you for sure find it like this just change the value in CE to 99 or whatever, and it works
highest you can set it is 998 (which displays 999) if your looking to freeze it
This wont raise your chain "level" though, just the chain number, so get to gold coins legitly first by not picking up loot then freeze it after, ill have to try to find a hidden chain "level" value now *sigh* lol

Edit:
20578970 is the drop amount/rates (chain level), max value you can set it to is 3, anything over 3 and nothing will drop. Other non usa versions can just start scanning at level 0 and go up one each time you get a "chain level" increase, also max value 3 though