Is it possible for XBMC to run standalone on for example an external hard drive?
I have got two laptops at home and would like to access the same XBMC database with both of them, without having to switch to the laptop I have installed XBMC on.
Try this: http://portableapps.com/node/19745
XBMC was really designed to be cross platform (i.e. portable) so its pretty good about keeping all its settings in a local file already.
You could use a MYSQL database synchronization plugin for XBMC or use Plex, which handles all of that for you.
With Plex, you run one server somewhere, anywhere, and you can login and access it from anywhere or locally.
You can add additional users that are also then able to access the same database from anywhere.
If you want control over who has access to what and where, you can do that.