It can be done with Hades Workshop[forums.qhimm.com], though you'll need to search a bit inside the game's script code.
It requires changing the script of all the fields where you are given the choice to change team (Hildaguard 3 and Invincible mainly), but that's not an hard modification. Search for the line "Party( 4, 1 )" and change it to "Party( 4, 0 )". This way, Zidane won't be locked anymore. Those lines can be found :
- in the Blue Narciss, in one of the Cid's functions (the SpeakBTN one),
- in the Hildaguard 3, in one of the Sailor's functions (the SpeakBTN one),
- in the Invincible, it should be in one of the non-model related functions, that is either a "Code" function or a "Region" function.

There are other moments in the game where you can change your party and you would need to do the change for each one of them. However, that's not such a good idea for several points in the story that require Zidane to be or not to be in the party (the Oeilvert/Desert Palace dungeons, for instance).

Here is a patch I made to make Zidane optional in the 3 places above for the US language.
https://www.dropbox.com/sh/ac7sr4q3z2cx9vp/AACQDfqXPvn8c3ylXeGUrBKEa?dl=0
It's the mod "PC_OptionalZidane.zip".

Thanks a bunch!

After finishing the game about 10 times in the original Playstation l got sick of using the main monkey in battle all the time for 500 hrs. LOL

Now l can finally kick him out to let him sit in the backline and watches the episode of Vivi / Dagger / Tin Knight unfolds.

Hopefully the Original Poster is still subscribed...

The dropbox link posted previously is no longer working and was wondering if you could reupload/repost

Updated the dropbox link (dropbox changed its politics about public links):
https://www.dropbox.com/sh/ac7sr4q3z2cx9vp/AACQDfqXPvn8c3ylXeGUrBKEa?dl=0

It's the mod "PC_OptionalZidane.zip".