I recently had exactly the same problem as you. It seems that if you enable ActiveExtensions[] in your siteaccess site.ini it works as intended. However, if you exclude the ActiveAccessExtensions[] the extension is not loaded. Seems to me that the whole extension loading thing should be revised as it is currently very confusing. I don't really see the point in having different settings for siteaccess/xxx/site.ini and override/site.ini anyway.
This enables me to put all the settings and design inside the extension without messing up the admin. This also enables you to keep all settings that are shared between all siteaccesses except the admin in one place which is very useful for sites with different siteaccesses that are really only translations.
Thank you. Yes, I had read your and other posts related to this problem. I tried the technique you suggested--essentially adding the variable ActiveExtensions, which is designed to be in override/site.ini, to siteaccess/.../site.ini, along with ActiveAccessExtensions. But the site.ini settings (site design, site name) in the extension still don't take effect, even after clearing the cache.
The other settings (override.ini, design.ini) continue to be effective, as they were before.
Strange, it works for me. Have you tried to remove the settings from the siteaccess/<yoursite>/site.ini.append file to see if the extensions file settings are used?
You can also try the ezini operator directly in a template and "fish" for some settings to see what you get. One thing you should be aware of is that the admin ini settings inspector will not be able to see the ini settings for your siteaccess correctly when some of them are loaded from an extension when the extension is enabled in this way.
You were all very helpful. In short, I had expected my extension's ini settings to take precedent over everything except settings/override; in reality, for a single site-access extensions, it has much lower priority. Thank you!