Government archiving rules often mandate use of digital signing. In the US, NARA has rules about what count as "trustworthy records" along these lines.
Doesn't "plain text" have a standard "container" in the form of MIME that also allows attaching things like signatures in a standard format?
7-bit ascii has been around for a lot longer than these formats which mostly represent a dump of the state of a formatting engine into something like XML in a file tree and compressed with PKZIP.
I was thinking of something like this when I wrote my comment. MIME does function as a container model, but it's not plain text and it could only be a standard with supplementation. But there's a UNIX-y sense in which this is all human-readable text representations.
I'm not really familiar with the internals of ODF, but OOXML, while it does capture quite a lot of UI state, is actually quite clean and represents the intuitive document structure rather well.
E.g., see http://www.epa.gov/records1/faqs/rks.htm
OOXML, PDF, and ODF each have a standard "container" and key management model that makes this a whole lot easier than plain text.