Well, Plus! increases the limit to 1000 chars and SPNG (iirc when I checked it out last a few years ago) used to set it to 5000 chars so clearly, it can be done. But doing it via the Plus! scripting interface would mean that there should be API provided for it, which isn't available (for a reason).
Patchou stopped at 1000 chars because the limit exists to stop over spamming which is the reason why the WLM team has the limit in the first place only, their limit is far too small. That's why Plus! and SPNG increase the limit but still keep a decent sized one.
Having even bigger messages has effects which are not desirable
/ 
