Yeah it's sorta already coded into the game with that relic effect, but that item is not legally acquired until much later in the game, also needs a Moogle, and uses up a relic slot...
That makes it one of the more tedious variations on the theme then (as opposed to simple late game/collectable option or consumables) but in most cases there will be a flag in the game that using the equip sets. You would then make a cheat to hold this hack at that, which you could presumably spin into a hack (the menu stuff is a bit harder) or possibly another more amenable item that does not burn a possibly valuable item slot if said item flags are all encompassing (if they are binary effects then it is easy enough to kick a 32 bit value or something to each item and have 32 binary or stackable effects) rather than specific routines within the game.
Anyway two main approaches
1) Conventional cheat finding.
2) Analyse the battle determination routine (or the equip/consumption routine where applicable).
1) is much as it sounds and like you would find infinite health or something you would do this. Binary flags are often annoying to narrow down as simple turn on, check for change, turn off, check for change... so I do encourage a leaving it alone, wandering around and doing everything (or at least lots) but change the state and then check to see what remained the same. As far as do everything but then some emulators will even have functions like new branch taken but I am not aware if any of the systems Final Fantasy 6 is on have emulators with this functionality, but at the same time I don't know SNES debuggers.
2) will likely see you play with some assembly, and not necessarily the nice and gentle tracing session introduction some like as an intro to it all, though still a good learning experience which grants the usual "if it is the thing that will see you learn it then go right ahead". Two main methods for final fantasy style random battles (obviously monsters on map in other games is different) I would look at first are each step has a random chance of activating (possibly cumulative or increasing chance each step) or at each change of map location/return from battle a random number generator fires and it will be that many steps/seconds/frames before the next battle. I don't know that I have ever seen a combo approach of random chance and step counter (any step has 10% say but after 20 steps/20 seconds you will be in another) but it is well within reason and I don't look at that many games for this sort of thing.
For the step counter approach then you could still find that with a cheat search (walking one square back and forth will not see much change other than game time counter and the step counter itself (whether ascending to a value or descending to 0 I would not like to guess at... generally older systems don't do great with descending logic but it is by no means an oddity or that hard that it would be strange to do it). Whether you want to guess a step counter and try possibly in vain to find something that does not exist is up to you. Once you find the counter make a cheat to hold either the step counter low (if the game uses steps for another purpose this might not be ideal) or expected steps at 999 or whatever (the average person using this sort of cheat likely not making it to 999 steps between towns and dungeons or other map changes). Alternatively follow it on up and see if there is a flag somewhere from the item equip it uses in this.
For other games with slightly different equips there can be different effects used (such items will usually have in game fluff like "drastically reduces chances of encounter, save for bosses", reduces encounter rate by ??%, as well as more subtle ones like only weak creatures/only strong creatures) but that is getting off topic. Barring the bosses only fluff if you do see such things it tends to be an indicator of a chance calculator rather than counter but not always.