[lhridley]

OK, I was able to answer my own question.

The issue had to do with the context of the tweet; if the tweet appeared to be one that had already been sent through the API, then it did not post, which is why replies to posts weren't showing up when I hooked into newpost_complete.

The solution: add a date/timestamp to the tweet.

I modified the tweet contents to contain:

$tweet .= date("F j, Y, g:i:s a");

as part of the $tweet string, which generates a date/time stamp in the format of Month day, Year hour:min:sec am/pm. This differentiates the tweet context enough so that a new tweet is posted.

Then, attach the plugin to newpost_complete and newthread_post_complete and you get posts for new threads, and replies to new threads.

Cool!