Hello,
Where and how exactly are you using the dummy key?
I just tried it with menu item as well as custom document type (it does not really matter) using the key above and it is working fine for me. I tested it using repeater and I have set the "Cache minutes" system setting for this web aprt to be 5 minutes, unchecked the "Use default cache dependencies" and add a touch key, in lowercase:
nodes|sitename|custom.documentype|all
and it is working fine for me. Once I change a document of this type, refresh the page - the cache is updated and new information is available on the live site right ahead.
Have you tried using the cache debug?
Best regards,
Juraj Ondrus