Email filters and autoresponders are a bit convoluted to expect wide adoption. A TXT record might work though. Is a referral in a TXT record any worse from a security perspective than a referral in an MX record?
Owning the domain is not the same thing as fully controlling the web service.
My use of Tumblr as an example rather than simply stating hosted-CMS has probably misconstrued my point. (I use Tumblr with my own domain and forget that it is primarily not used that way.)
I hope the needs of providers like Fastmail (who provide services at users domains) are top of mind for the people behind this project as it's difficult to see it getting traction if it can't be deployed en-mass by hosting providers.
At the risk of stating the obvious choosing webfinger implicitly excludes anyone who does not or can not control their website sufficiently to implement webfinger. (ie: hosted webservices similar to Tumblr et al)