17/10/2008 - mIRC v6.35
5.Added custom dialog support for "rich" editbox property that allows
an arbitrary "limit" on the amount of text.
If you want to use Bold, underline, colors, etc, in a dialog, DCX supports these. MDX might also, I don't know. Of course, you can also wait for the next version of mIRC, which I believe is in the testing stages.