Sounds like a kind samaritan could write a scanner to find as many RSS feeds as possible which look like RSS/Atom and don't have these content types, then go and patch the hosting software those feeds use to have the correct content types, or ask the webmasters to fix it if they're home-made sites.

As soon as a majority of sites use the correct types, clients can start requiring it for newly added feeds, which in turn will make webmasters make it right if they want their feed to work.