autoselect GPG key for signing by group

[Hikaru Ichijyo]

I would like to sign/decrypt messages using one key in some groups,
using a different key in other groups, and in still other groups, not
sign/decrypt at all.

I have most such per-group preferences for other non-encryption related
things setup in my ~/.gnus.el using the gnus-posting-styles mechanism,
but since that seems to mostly just set variables (and not run
commands), I'm not really sure what I would put there to do something
like this.

Most people seem to invoke GPG signing like this (assuming they only use
one key everywhere, always):

(add-hook 'gnus-message-setup-hook 'mml-secure-message-sign-pgpmime)

How would I setup my key selection to depend on what group I'm in?

-- 
He that would make his own liberty secure must guard even his enemy from
oppression; for if he violates this duty, he establishes a precedent
that will reach to himself.
                                        --Thomas Paine