don't think its legally enforceable at all ... and why should it be?
ride within your skill-set and that includes an unexpected object whether it be a walker, dog, wildlife, rider or large rock, branch.
the nature of the environment we ride in has variables and the challenge for us all is to adapt to them.
so what if someones climbing up, that's mountain biking you can whinge all you want but at the end of the day you can't assault them (well you could but that might get you into more strife).
the only way to guarantee a controlled environment is to do a competitive event, that's marshaled.
I enjoy the challenge of going around a climbing rider, we need to put that into your skill-set otherwise mtbing is going to be an angry experience for you.
and remember the climbing guy might be a super fit bad bastard who will bite back if you tempt him.
in legal terms; if someone was injured the question would be as to which rider showed responsible duty of care and you'll find being out of control or unable to stop is a failure of your duty of care as the environment has these dangers to contend with. guess you could argue that x rider should have seen sign, (maybe he came in after sign), so onus is back on descending rider to ride safety. would be similar situation if you hit walker going up track or stopped rider who has crashed on track or is stopped mid track.