Anti-Global Illumination Volume

[KVogler]

so that the immediate entrance will still have some global illumination and disappear over time

Sound like you are describing eye adaptation… That already exists…

(Such as a well that goes down 30 feet before it opens up to a cavern - - none of it would require indirect lighting beyond a certain point,

So you just exclude that part from your lightmapp importance space.

so everything in the Anti-GI Volume would be pitch black with the exception of direct lighting (Such as the sun at high noon shining further down the well than at any other time).

But then the illuminated spot would cast, realistically, diffuse this light into the cavern, so making it not pitch black.