I edited the OP to reflect the updates I posted as a draft earlier.
Would it be OK to have a signature linking to a blog/article explaining what mixing is, how it works and containing URL link(s) to mixer(s)?
It'd be OK in a post. In a signature it would probably not be allowed, as it would look like "promoting a mixer".
theymos, what about this?
Also I should point out, it should not be ban-worthy to link to a closed mixer's ANN thread. Or actually to any post in the ANN thread that is not advertising to use the mixer.