I like the modification of the defualt messages mIRC uses.
A simpiler way would be for mIRC to use a (mirc.)INI file or so, then retrieve the messages from there and parse it accordingly.
Example:
mirc.ini
[messages]
onjoin=%nick% has joined channel %channel% (%addr%)
onquit=User %nick% has quit (%addr%) (%text%)
oninvite=You have been invited to %channel% by %nick%
onnick=%nick% has transformed into %nick2%
For those of you who use the 'mIRC Stats' program, something similar to the parse file would be addequet(sp?) for this suggestion.