Scott, I am going to offer you another perspective to the question you ask.
In a product like Blackboard, your need is extreme because Blackboard does not offer any way to grade individual posts (as I recall). For example, in Blackboard, if you want to know how many posts were made by a user you might manually count them.
Moodle, on the other hand, provides the ability to "Rate" (or grade) each individual post. Additional, you can pick from a variety of tallying options, such as "Sum of the ratings", "Count of the ratings", "Max..", "Min", and a few more. So, for example, if you are interested in tallying "total posts", this is simply done by giving each post a rating of 1 and choosing "Count of the ratings".
Now, this doesn't help with knowing the "average, minimum, or maximum post lengths" as you requested. But I have a personal philosophy about this. I believe that the quality of a post has little to do with the number of words or characters, so I never use this type of criteria. Instead, I tell my students that I read every post (something that some instructors don't do because they say it takes too long) and then I give them between 0 and 2 points. For a student to get a perfect score (an "A") on discussions, they have to achieve a total of 8 points. Then, in Moodle, I use "Sum of ratings" to tally their scores.
I find that by reading a post, in context, is the best way to judge what a student is saying.
Of course, none of this can be be done in a product like Blackboard, so one has to resort to a method of simply counting characters or words (which also can't be done in Blackboard).
I certainly understand your question and needs, and don't take any of what I say as critical, I am only trying to give you another perspective that might help you solve your problem. Maybe you already know about Moodle's rating feature, but maybe not.
Good luck.
(By the way, I do use the technique that Marcus suggested after a discussion is over and I notice that a student might have a low score. I wonder, "Did this student really not do much, or did I forget to grade one of the posts?" So then, I go into the Advanced Search feature to explore how many posts a student made. If I notice that I forgot to grade a post, it is a simple matter to click on "See this post in context", and then provide an appropriate grade. I find the Moodle feature to be extremely useful.)
