Protocol for downhill cyclist overtaking a car?
Please accept my apologies on behalf of cyclists. Hollerin' something at a motorist who was trying to figure out how to handle an obviously unclear situation was inappropriate. Thanks for doing your best and not killing any cyclists that day!

In general, I agree with the other answers here that you handled this fine and there isn't some magic you could have worked to make the situation go away. Having frequently cycled one of the curviest and steepest highways in the US, I have often been in the position of your cyclist. I also know from a drivers perspective of the same road that there are not a whole lot of obvious moves.

The primarily impetus for any action should probably be on the cyclist. As a motorist being aware helps a lot; but don't try to do anything fancy. Keep it simple. Watch for what kind of move the cyclist wants to make and adapt, but be consistent yourself. He's trying to guess how your car is going to behave -- if you change patterns on him, all bets are off.

The easiest place for a cyclist to pass is usually the place where cars naturally go the slowest. This means curves. Straights are hard because cars speed up (and try to pass each other). While we do have to follow the rules of the road, passing on a double yellow curve is sometimes the safest thing to do on a bike. Inside curves (left turns for US folks) provide the most visibility and a chance to cut a shorter path than the car. The best thing you can do is stay all the way in your lane and take the curve easy. Let the cyclist make the move. You don't have to be in the shoulder or pulled over or anything, just keep your own company and don't wander all over the road yourself.

It's ok to brake. Cyclists have brakes too and there is no physical reason we are forced go fast or follow close. If you want to be extra nice, when you do brake, touch the brakes lightly at first so your brake lights come on early and gradually add pressure to slow the car. In fact, if a cyclist is following you too closely and you are uncomfortable, tap the brakes a couple times to signal you need room to brake. You don't have to actually brake, but the brake lights are the best signal you can give to indicate you want the cyclist to add some distance.

Turn signals are confusing, but usually a right from you means you are going to pull over and a left means you can see the road is clear to pass. However, these are not set and stone and the cyclist still needs to make a judgement call. Hazard lights are even more confusing: I would save them for indicating true hazards.

To make a long story short, drive normally and safely yourself, and let the cyclist take care of the passing.