I've just set up a new site where I want to do exactly the same, but enabling the extension through ActiveAccessExtensions[] doesn't work. Settings in the extension are blindly overlooked :(
I still have exactly the same problem. I want to keep my work separated within extensions so I work in an extension activated on siteaccess level by setting it up within settings/siteaccess/<my_siteaccess>/site.ini.append.php. I used ActiveAccessExtension to activate it there.
It worked so far, but as soon as I tried to override the imagelarge alias via extension/<my_extension>/settings/image.ini.append.php it always get overriden by the one defined in extension/ezflow/settings/image.ini.append.php. Therefore the image settings within ezflow do not have the characteristics of an overridable setting. The only way I was able to override it was to construct a global extension with the specific settings and to load it globally within the settings/override/site.ini.append.php file or to put it directly into settings/override/image.ini.append.php.
So currently it is not possible to override extension settings, which should have the characteristics of default settings, on siteaccess level. Am I correct?