Check out the Media Bay. It essentially does what you were describing. You can mark as many folders as you like on your entire system so they are constantly scanned for HALion compatible content.
Content included with HALion/Sonic/SE typically goes in the same location as long as you are consistent in choosing “install for all users” or “install for just this user”.
On windows machines it goes into “%SystemDrive%\ProgramData\Steinberg\Content”. In some cases you might find content in your %USERPROFILE%\AppData\Roaming\Steinberg" folder, but you’ll also find a shortcut in the directory above so it is easy to track down.
If you don’t want stuff on your system drive, just move the Content directory to some other drive and put a system “junction” from the original location pointing to the new location.
User content can be managed with the Library Utility…which allows you to put the content anywhere you want.