It seems that sharing across contexts is good from the core perspective (element/plug-in creation etc.) but from the page/include/front end standpoint is a little tricky. Im doing the same thing (except with 4 unique domains). The problem was that within each context the base urls & directories are independent from the install directory so the paths are localized to that domain.
Instead of having multiple asset directories and such (which kind of defeats the purpose imho), for each context I created a new context setting to override the default url of the asset directory.
So under context, I added a new context setting key called "assets_url" and just filled it in with my url to my main install directory (eg:
http://www.foo1.com/assets/) and then I call it like this within my template chunk.
<link rel="StyleSheet" href="[[++assets_url]]css/global.css" type="text/css">
I’m not sure if I could use a manager_base_dir or something like that (tv variable perhaps?) within the context setting so its not hardcoded to the url but Ive only begun playing around with revo and Im still getting my sea legs. I guess my logic is that its just rewriting the assets directory in the config file so I only have to worry about one asset directory. You could also probably accomplish this with a apache rewrite directive of some kind but im not sure if thats the most effective to go about it.