Coded gesture for "I am looking for a place to stop"
I have this same problem on a couple of local descents.

There isn't signal that communicates what you want to say, clearly and succinctly.  Anything you try and do will take at least a hand off the bars and reduce your overall stability at speed.

Here's some options:


What you can do is stop pedalling, and pull to the side of the road so there is as much space available for the vehicle to pass.  THEN its up to the driver of the vehicle to decide if its safe to pass or not.   A car will need some hundreds of metres of clear space if they have to cross the centerline so don't bother if you're anywhere near an upcoming corner.
In fact if you're getting close to a corner, its sometimes better to clearly take the lane by shoulder-checking and then moving into the lane. This stops anyone from passing you in a dangerous way.
If I absolutely needed to show I was slowing to allow a pass, I'd probably do something like pull to the side as far as possible, maybe unclip my outside pedal and have both legs straight down.   If the shoulder was wide enough I would be very much off the roadway.  Downside is most hillsides lack in road shoulders so there's not a lot of space.  You can also try to look at the following driver and establish an eye contact - but they might think you're simply challenging them or something.
Lastly, another physical way to show you're stopping is to straighten your arms, lower your torso, and get off the saddle.     This will put your backside backward, and is a pretty clear indication that you're braking hard.  Mostly visible to other riders, someone in a car might not perceive it as clearly.


In some places, the painted centerline may stop the vehicle from being allowed to pass, so you might be wasting your time if the road is narrow.

Your other options are to get going faster, or to time your descent so it starts at the end of a car column with a space behind you.  This can be aggravating if the line of cars is slower than your descent. 

HOWEVER...

At all times be safe on a descent.  Don't feel you have to push and outride your comfort zone/speed.  There's a lot of kinetic energy when descending, and if that transforms into friction or a sudden stop, then its a bad day.