From what I can tell, the .tex file loads one of the other three: \loadmarkfile{lang-url}
\loadmarkfile loads either lang-url.mkii or lang-url.mkiv, depending on the ConTeXt version you're running (MkII / MkIV). In Mark IV, the Lua code is then put in lang-url.lua, which is input by lang-url.mkiv (you can see "\registerctxluafile{lang-url}{1.001}" near the beginning of the latter). This architecture enables you to reuse the Lua code in completely different environments (for example, in a pure Lua script).
Our project has a requirement of using Xetex, so I have to stick with that. Does that mean lang-url doesn't work at all?
ConTeXt on XeTeX is considered Mark II as far as the mark business goes (it doesn't know about Lua), so you have access to the exact same code as with pdfTeX; in this case, lang-url.mkii will be loaded. But if you know that all your users will be using XeTeX, you don't really need to worry about the \loadmarkfile mechanism; it is there to accommodate different engines.
We also have users using ConTeXt Minimal as well as ConTeXt from the TeXLive 2008 distribution.
The particular distribution one uses shouldn't be a problem at all for implementing hyphenation rules.
I want to do things in a way that will work in both. I would be happy to put in some extra effort to make the result generally available to others who want to follow the Chicago style for wrapping long URLs.
This will certainly be most appreciated. Arthur