Additionally addon installation would want to automatically target a specific site, rather than the base files. But perhaps entering the master password would allow you to override the behaviour.

Generally, use of the master password to unlock admin features may be another good solution, rather than trying to sandbox. Or maybe, in addition.

This is why in Drupal, for which I am accustomed, all muilt-sites are placed in the core sites folder and specific site related addons go in each of the site folder as do templates and the all important settings file which directs to the database. This way the core features on the CMS are easily updated without affecting the individual sites then each individual site folder and database can be auto updated with drush. Actually all of it is updated with drush. Drupal like Composr has multisites where sharing database and users but across different sites as well as keeping them separate without the need for new installs for each site. Finally, I always use the same user and password to connect to all databases, being the root user of my VPS and simply switched in mysql to the one I was using. The settings file was happy to accept same user and pass for all as well as same admin and pass to access administration for each.

If like me you run many sites of your own for different things like I have my art portfolio site, I have family sites I've made , I have business related sites and testing project sites as well as archive sites. The ones that are created on a cms I'd like to use the one cms and not as I have it now some drupal sites, some joomla and although I do really love drupal now I am making sites for others, it is not so easy for people with no experience to configure their own site in drupal. I have friends that want to share my vps space which makes sense because it helps me with the cost of it. I need to set up one cms for multiple sites. Composr is I am told the winner this year for best cms!

Thanks for the insights :)

"Addon segregation" will now be a lot easier due to #3326.

I do want to note that the multi-site-install functionality (aka Demonstratr) is not the only way for Composr to support multiple sites via one install. You can also bind zones to domains, which is a lot simpler and supported out-of-the-box.