Called from thread action menu before returning a 'send mail' link for the post author. Not all authors will be able to receive private mail, for instance those on other networks with incompatible mail systems.
By default author_is_pmable() returns true for 'zot' xchans, and false for all others.
The plugin is passed an array
[ 'xchan' => $author_xchan, 'result' => 'unset' ]
A plugin which sets the 'result' to something besides 'unset' will over-ride the default behaviour. A value of true will enable the 'send mail' link and the private mail recipient will be set to the author's xchan_hash. A value of false will disable the 'send mail' link.