A marvelous extravagance just arrived from England: bar-end turn signals by Oberon. Beautifully machined aluminum bodies.
Here's what they look like installed (on a different bike). They're two sided, so they're visible from the front and back. I hope they make separate rear signals unnecessary. There's a state inspection station nearby that's also a custom bike shop. They'll be able to tell me if I'd pass inspection.
They wouldn't really work with buckhorn bars, but I'll be going with clip-ons, which also makes installation really easy. I won't need to drill the bars for wiring, just the plastic caps.