All the settings currently available to you via Group Policy are listed on the Chromium site. You can block all extensions, then allow certain ones via a whitelist, but I don't see anything in there that does specifically what you want.
In terms of blocking access to extensions, they will have access to whatever the user running Chrome has access to. Since the extension can access the content after it has been downloaded by the browser, there is no way the server can control this access.
So your choices seem to be maintaining an approved list of extensions you aren't worried about, or making a custom version of Chrome (Chromium is Open Source) to distribute to users that includes the functionality you are looking for.
Reading the Chrome Extensions Developer Guide section on Content Scripts, particularly the sections on Execution Environment, Communication with the Embedding Page and Security Considerations, will give you a better understanding of how Chrome Extensions work, and may suggest where you have opportunities either working with Chromium, or customising extensions.
Or set up some sophisticated deep packet inspection firewall that can identify content you don't want to leak, which would have the additional benefit of covering all applications - but be rather expensive :-)