Request for HTML5 RegisterProtocolHandler() to support HTTP POST

I am unsure whether it is a limitation of a browser extension I recently
used or an implementation-type enhancement that would prevent this,but my
goal for the future web being able to realize it's greatest potential would
be achievable if mailto: links could be handled in conjunction with the
RegisterProtocolHandler in the subject specified way.  Although, my perhaps
very evident failure to have researched prior to sharing this may deem this
a moot suggestion at the outset, I thought I should comment here (in
addition to the official spec comments page @bugzilla to ensure
visibility)....

The request to accept HTTP POST requests in conjunction w/
RegisterProtocolHandler() is based upon the truth that HTTP GET requests
are limited in length by the cap: URL.length (~256? chars) while HTTP POST
is not.

Future direction: I should contact the developer who published the app
(ExportTabs) to see if I can resolve by asking they can revise based on
HTTP POST request somehow, but in my testing for mailto:, I was unable to
construct a new email message via an HTML(href="mailto:..")  that
encapsulated my email message into the *register*'/handled newly created
email.