This kind of override would be very nice, there is discussion on this on the issue tracker but I can't find back the issue, if someone remenber...
Currently for this problem I use the ActiveAccessExtension settings which allows you to load an extension for a specific site access. Thus, I can load one extension for front end siteaccess and another one for backend extension and I put my override and front end settings there.
I load the generic "frontend" extension which contains generic settings
In my siteaccess, I can override the generic settings from my "frontend" extension
On my secondary siteaccess (for translation purpose) :
I load the generic "frontend" extension which contains generic settings
In my siteaccess, I can override the generic settings from my "frontend" extension
On my admin siteaccess :
I load the generic "backend" extension which contains generic settings
In my siteaccess, I can override the generic settings from my "backend" extension
So if I undertand well, it leads to something like settings/override/* > settings/siteaccess/<town>/* > extension/<my extension>/settings/* ?
It reminds me that sometimes there were troubles with settings within extensions. Is it now safe/solved (we use 4.0.6 so far, should use 4.3 by the end of the year).