Hmm, stuff with HTML is not that easy to truncate cleanly. It's certainly possible, but the server might have to generate the appropriate end tags. That means at least porting or adapting code from some other place (the post preview function might be a good place to start)
On the other hand, the more annoying a post in NTC is, the faster it will get required votes to "Edit" or "Reap". That is also something to keep in mind. That's the trouble with a nice, friendly place like PM. You can't "admin abuse" a troll unless the community gives you the go ahead.
perl -e 'use Crypt::Digest::SHA256 qw[sha256_hex]; print substr(sha256_hex("the Answer To Life, The Universe And Everything"), 6, 2), "\n";'