Except for a blog post here and there, describing the custom attributes someone created, but that do not seem to get any traction - like one describing how to enforce immutability, another one on Documenting Thread Safety, modeling the attributes after JCIP annotations - is there any standard emerging? Anything MS might be planning for the future?

This is something that should be standard, if there's to be any chance of interoperability between libraries concurrency-wise. Both for documentation purposes, and also to feed static / dynamic test tools.

If MS isn't doing anything in that direction, it could be done on CodePlex - but I couldn't find anything there, either.

<opinion>Concurrency and thread safety are really hard in imperative and object-languages like C# and Java, we should try to tame it, until we hopefully switch to more appropriate languages.</opinion>