We could just create our own standard and hope it eventually gets enough use that it becomes a de facto standard that everybody has to support. As long as non-compliant readers ignore it properly, you're golden.
In fact, IIRC, the EPUB 3 spec has this weird scheme where you can reference arbitrary metadata from external standards. Although I swear every time I have to waste two lines of XML just to insert an ISBN, the syntax (awkward as it may be) theoretically lets you insert just about any arbitrary metadata you might want, e.g.
Code:
<dc:language id="foo">en-US</dc:language>
<meta refines="foo" property="original-language"
scheme="mobileread:stuffdcforgot">martian-Mars</meta>
Not that any reader would do anything with it, mind you, but if enough people picked the same scheme, maybe eventually somebody would.