AppSpot, even if used as a proxy is very likely not caching your page. (That's even what the denial of your DMCA says.)
It's very likely the user is running a very simple script and "grabbing" the pages from your server on-the-fly, then showing them.
I don't quite get where you are coming from. I understand the basics of a proxy and how it pulls data from one source to display to another. If Appspot subdomains are indexed in Google with my client's homepage, how a user accomplishes this should not matter to the owner of Appspot (Google) when they receive a DMCA notice.
You didn't ask them to remove the appspot subdomain and cached copy from their search results.
I made reference to it in the original DMCA notices. The Appspot caches are dropping, albeit at a slow pace. Whether this is related to the original DMCA notices or htaccess restrictions is uncertain. I also responded to their denials with specific references to the cached versions of my client's homepage. No response has been received from those yet.