This sounds like it could be a useful feature. It may be a good idea to include options for it though. "Enable/Disable" and "Auto/Ask" come to mind.
Yup, I said that an option of such would be nifty to have.
About your script, genius: It would be nice if it also worked somehow in actions (/me) and query windows as well. The script also bugs quite much. It works okay if you don't go over the point where there's too many characters in the editbox and mIRC automatically posts what you've written. If that happens, the script just refuses to work after and your lines don't get seperated to smaller lines anymore. The script just posts something that's left in the buffer, defined as some variable, I guess. Needs a little bit of fine-tuning.
It would also be nice if it added ellipsis (...) in the points where it cuts, before and after.
I completely suck at scripting, so I don't know how to fix it.