There's a guide to getting it set up still alive at http://www.csn.ul.ie/~caolan/TechTexts/Broadway.html
This was in 1996, in the X11R6.3 release ("Broadway"), the same release they came out with LBX, the low-bandwidth extension to X, so that you could run your X11 apps over a modem with reasonable throughput.
It never got much adoption. I don't really know why. My guess, though, is that it didn't address the real reasons people were moving from X11 to browser apps: server load, security, ease of administration, latency-tolerance, and ease of development.
Today, server load and latency tolerance are a lot less important than they were in 1996, because computers are a hundred times faster and hundreds of millions of people have ping times under 20 milliseconds, ten times faster than you'd get on a modem.
But security, ease of administration, and ease of development are a lot more important now than they were in 1996.
So, I suspect this won't get widely adopted, but it could go either way. I hope it does!