Anyone have any suggestions on how to adjust this headset? With the brake cable hanger/stop and locknut in place, there's no room to get a wrench on the adjustable cup--the stop is in the way.
I could wrap some electrical tape around the rim of the adjustable cup and grab onto it with some smooth-jawed water pump pliers, and hold the adjustment while I tighten the locknut. But the real kicker is that there's no keyway machined into the steerer and no matching tab on the brake hanger to keep it from rotating with the locknut as it's tightened. I'd need to hold the locknut wrench with one hand, the pliers with another hand, and keep the hanger from turning with my third hand.
How to deal with this? And why did Raleigh think it was a good idea to leave the steerer unslotted? Or was the steerer-slotting guy just having a day off?

You just need to hold the brake hanger. If it doesn't turn, the cup won't either.

I have smooth jawed channel lock pliers I use for holding the hanger. It usually takes a few tries to get the adjustment right since there is a sort of skewer effect, like when you leave a little play in a hub and let the skewer take it out.

Well, thanks, guys, that worked--should have thought of it on my own. I've never encountered an unslotted steerer before, though. Is that at all common? Seems to me more like a manufacturing oversight than a design feature, but maybe I'm wrong about that.

Unslotted steerer tubes are very, very uncommon. With a setup like that all you can do is what was previously mentioned. It helps alot if you dis-connect the front brake cable as it reduces pressure on the hanger.