Looks interesting, but after activating Native Client, I just see a black box in Chrome 15.0.849.1 dev on OS X 10.6.8 – anybody else has that issue? Or do the games only work on Windows?
I also get a 403 trying to load what you've linked, but dosbox_13.nmf loads fine and so do the games.
It determines the filename based on the major version number in the useragent - it would probably work fine if your browser reported chrome 13.something.
I was on 14.0.835.2 (Win7) and had a similar issue (black box, and very laggy scrolling). Once the GPU compositing and GPU accelerated drawing options marked out here [1] are disabled, the issues went away.
Looks like the dosbox is loaded depending on which version of chrome you're running, and there's only 12 and 13 up, so 14/15 tries to get a file that doesn't exist.
Bit of silly coding, given the ABI is fixed from 13 onwards.
I wonder the server is refusing this request if the user-agent is not in their white list?
Or perhaps they are limiting it based on Country due to Copyright Issues?