@Ted Clayton: I'm surprised your tests worked; SCE clearly builds a custom comment form and does not invoke any of the regular comment form hooks.

(Those hooks would fail to operate, as SCE's comment data structure is vastly different to WP core's comment data structure [during comment form processing].)

Off-hand, I'd guess that the plugins you tested are all related to listing/viewing comments, but do not enhance or customize the comment form. :)