Some people have asked for this on the old forum. If it were implemented I would keep it turned off though, just my personal preference. I do the same with websites. I code all webpages in B&W Notepad instead of using a specialised HTML editor like Editplus.
Implementing syntax highlighting would certainly not do any harm though and I believe such a feature would be popular.
a better alternative would be the oft-suggested external editor, so we could use xemacs or textedit or whatever to edit them, and then when the editor closes or saves the file (which can be monitored iirc) mIRC auto reloads it...