ZDoom

The problem here is how KEYCONF is implemented. It's an either-or proposition. The feature is a mess and the only way to be safe from it is not to use it and tell people who still do that they do things wrong.

I was hoping there could be some sort of flag, a MAPINFO flag in particular or perhaps a weapon flag where, if assigned, it's always set to the slot regardless of keyconf.

Or is there no way to do even that?

Can't be done. The only way to make it less invasive would be to outright disable it. And no matter what I try - anything would outright break any old mod using this method. This had been deprecated for ages, for exactly the reasons you state. But I am powerless when people still use it instead of better methods.

If there's one thing I'm currently experiencing some frustration with, is the fact that keyconf overrides EVERY METHOD when using setslot.

Now, I could override the player's PickNext/PrevWeapon as I'm assuming those deal with "weapnext" and "weapprev", but that seems like a bit of a crutch. Simply put, I'm trying to find a way to add weapons on top of those that have 'setslot' used upon them. It'd be appreciated if there was a way to interpret them as 'added on' instead of 'this and only this'.