I'd like to have a command I can insert into a command pipeline that adds color escapes to its input according to vim's syntax highlighting capabilities.
For example:
cat somefile.js | vim - <???> | less
The resulting text would be that of somefile.js, but colorized according to how the current vim configuration would do it in-editor.
It occurs to me that this must be possible. I agree that the example up there isn't what a sane man might call exactly useful, but that doesn't mean the idea never is.
I think your idea has one basic flaw: that nobody ever thought about allowing such a thing.
Clearly vim is capable of doing syntax highlighting. But I'll bet you an ice cream cone that if you can manage to get vim to stream text through and process it, that you won't like the results.
Consider what happens when you pipe text through more (or less if you prefer). When it goes to the terminal, these programs display one screenful and wait for you to hit the space bar. But if you redirect stdout to some other place than the terminal, these programs notice this and simply copy their input to their output unchanged.
If vim doesn't notice that you are piping text through, it is likely to send cursor-movement commands that you probably don't want in your output. If vim does notice, it is likely to just pass the text, and not syntax-color it. Only if vim does do the syntax-coloring but does not inject cursor-movement stuff will your idea work.
You could try it. Here's an answer that discusses piping stuff through vim:
Execute a command within Vim from the command line
But I say why not pipe your text through a program that was designed and intended to have text piped through it? Pygments can colorize every major programming language and markup format.
The major advantage I see for your idea: you can customize the way vim does syntax coloring, get it the way you want it, and then also use vim to process your text. But it's probably not that hard to customize Pygments, and it might even be satisfactory out of the box, in which case it would definitely be the easiest way to go. And Pygments not only has ANSI sequence output, it also has HTML output, RTF, LaTeX, etc. So if you get Pygments working the way you want it to, it should be able to output whatever output format you need; vim will only have the ANSI sequence one.