Out of interest, we have even had stones lobbed down the shaft whilst the BPC's GG Winch is running - with the chair carrying punters up and down and a crowd of folk around on the surface (though culprits admittedly usually kids/ teenagers). But you do wonder how dumb some folk can be!
So, for me I go nowhere near the foot of the shaft if I can avoid it.
I suppose you could leave a volunteer on the surface if doing the Main Shaft or Dihedral...
Another alternative would be to make & take your own sign which could show a date & time so people would know it meant 'currently'.
To be honest at GG, when the Winch isn't there and our safety netting's been removed, you are more likely to get hit on the head by some dickhead who leans just a bit too close over that loose edge or the mossy, wet lip.
Or a four year old kid being held at arms length by his Dad whilst being asked if he can see the bottom.
Trust me, I am not joking.
So, going down a deep hole?? - Be afraid, be very afraid....
Scoff
BPC/ CDG