It was an existing extension I tried to convert to ezpublish 4.0, so the settings were alright.
It turned out, the cache messed up my testing results. After some additional tests and much cache cleaning, I have it working now again. Initial reason was, that you have to move templates not mentioned in "override.ini.append.php" from "overrides/templates" to "templates" with ezpublish 4.0.
Maybe i'm wrong, but where you say
EXT_NAME/design/EXT_NAME/overrides/templates/full/foo.tpl,
shouldn't it be EXT_NAME/design/YOUR_SITEACCESS_HERE/overrides/templates/full/foo.tpl
or maybe EXT_NAME/design/MY_SITEACCESS_HERE/templates/full/foo.tpl?
You're right, the secound one should actually be the DesignName, but my extension and my design have the same name. Your location works for me too, up to and including ezpublish 3.10.0.
With version 4.0 some things seem to have changed. All override-templates (anything mentioned in override.ini.append.php) have to be moved to "override/templates". That's the part, that's mentioned in the upgrade guide from 3.10 to 4.0. In contrary, everything that overrides other templates from base (e.g. ezmatrix.tpl), I had to move from "override/templates" to "templates" in order to work without a pointless override rule like
