new_file_permissions setting post install?

I have an existing modx install (advanced, 2.1.1) that was installed with new file / folder permissions set to 755, but needs to be changed to 777. I tried to create a system setting for this, but it didn't seem to have any effect.

Just a little background in case it's relevant:

This install runs multiple sites. Originally they were all sub domains of the same site, then there were a few new domains added, etc. All of these were running fine as separate contexts, and modx was installed under one Linux user on a VPS (Ubuntu, FastCGI, PHP 5.3.2).

Unfortunately, some of these separate domain names needed SSL certificates, which meant they needed different IP addresses, which meant they needed to be run as separate accounts on this VPS. This was pretty easy to accomplish by moving the core directory up to the /var directory (out of /home/user1) and editing some core.config.php files, but the cache files are created / modified by multiple Linux users now. Since they are all created as 755, I'm worried that if a cache file is created by one user it can't be refreshed by another.

I just assumed new_file_permissions / new_folder_permissions would solve my problem, but if anybody has something a little more creative I'm open to ideas.

Thanks!