I think the concept of yielding based on primary trail direction is a good one but I do think you'd need more signage.
I always seem to come across hikers hiking up the steepest part of a trail while I am descending and I always do my best to yield but it's literally dangerous to try to do so.
It seems hikers like to turn up severed at the penny/GSM/severed intersection. The steep section before that is not a fun or safe section of trail to attempt to dodge hikers.
Not that I still wouldn't try to slow down and avoid them if the rule was changed but I really do think it's worth pointing out to hikers that they can expect mtbs headed downhill.
As soon as I see them I tell out a friendly "good morning!" Followed by "I'm sorry but I may not be able to get stopped at this point".
They generally make a face as if I am riding my bike through their flower garden in front of their house. Whatever. I try.
I feel like ebikers headed up would be the same deal. Silly for someone descending a steep downhill primary trail to have to pull over and stop for someone headed up the wrong direction.
If you really want to climb a descending trail, regardless of your mode of transport, you should be yielding.
And you do need local knowledge of there's no signage. For example o rode Alice lake a couple weeks ago and after descending crouching squirrel hidden monkey I went up brackentrail to get back to the main trail network.
I would not do that again and felt bad about it, once you're halfway up it's clear it's really downhill primary. I felt like a tool and definitely yielded to any descenders.
No way it's have done that if it was signed (and since that area generally is well signed and there are some gnarly steep climbing sections I didn't second guess it).