I am wondering if theres any way to change/use loading screens for custom maps.

thanks

if you name your map sp_a#_XXX, it'll use the cinematic zoom-out sequence for that portion of the game. E.G. the glados zoom-out for a1, the turret-box zoomout for a4, etc.

As for custom loading screens, it's not possible AFAIK without having your map load from another map. You can specify which loading screen the next map uses within the exit elevator instance, but not what the current map uses.

Could you be a bit more precise on where to specify the loading info in the instance?