This might be a bit of a weird one.

Does anyone know where in the game code it stipulates what outfit a Sim should wear when visiting a Nightclub Lot?
For the Gym there is a tuning file called job_excercise_gym. For the Pool there are several tunings called jobs_Venue_Pool_[agestage]. For the Lounge there are tunings called job_barfly_lounge. Altering those has worked perfectly well to alter what outfit NPCs and Player Sims wear when arriving on the Lot. I can't seem to find one that works for the Nightclub venue though.
These tuning names are as they are given by Sims 4 Studio, by the way.

I've found ONE that explicitly has Nightclub in the name but that is only for dancers. That one only seems to work if there is a DJ booth on the Lot. I tried changing the generic barfly tuning but that had unusual results. It made it so that NPCs would turn up to Nightclub venues AND Bar venues in their Partywear BUT Player Sims would only turn up to Bars in Partywear. They still arrived at Nightclubs in whatever they were wearing when the Travel option was chosen.

What I am ultimately trying to do is to get Active Sims and NPCs who visit Nightclubs to turn up in their Party wear instead of their Everyday (or weather related) wear. As you can tell, I'm not having much luck.

You mean this XML " jobs_Venue_Nightclub_NPC_Dancer"? It does say:

<T n="filter">123749<!--TunableSimFilter: filter_Entertainer_Dancer--></T>
<T n="goal_scoring">10</T>
<V t="uniform_specified" n="job_uniform">
<U n="uniform_specified">
<E n="outfit_change_priority">Medium</E>
<E n="outfit_change_reason">Category_Party</E>

I believe all Sims change to Party cloths or should. But it is only at Medium priority. So changing it to High may help. I know that Seasons Rain and Winter Clothes are overriding the other outfits alot in my game. There was a Lot trait Mod that allowed you to set how everyone dressed on a lot. I think it was by LittleMissSam. Don't quote me on that though. I don't know if any of this helped. But you helped me with knowing where to look. I wouldn't have thought to look under job. For this I thank you.

Yeah, that's the one I edited. I changed the priority to High and also added in a line of code that states the player Sim should also change outfits which I found in the gym tuning ( I think) but it had no affect on played Sims and I can only assume it's because that is not the role the player Sim is given when visiting the Lot.

I've had one other suggestion of something to try on another forum where I also asked. They have suggested cloning some job tuning that DOES affect the player Sim, make it custom, and then have the club venue zone director file use that role instead and see what happens. I'm yet to try it out though.