I'd like to serve up content only to my LAN using the existing uhttpd that's running LuCi. Is it as simple as creating a webroot under /www/foo/ and mounting an external device to that mount point? Am I missing anything?
Yes it is that simple. Though I would put a link in the /www directory in internal flash that points to somewhere on an external drive, so web users see only a certain part of its tree.
If I remember, the stock uhttpd is not built to serve a single file that is larger than 2 GB, such as a video.