you;d need to find a program which has the ability to do custom syntax highlighting. Then you'd need to customise it yourself, and probably input every single mirc command and identifier into it. I think there is a custom config for a program called "Textpad" which has that but textpad is not free so i havn't tried it.
But frankly, if you can write mirc script you can read it just as easily without bothering with syntax highlighting. However i still wish the inbuilt editor had syntax hightighting, that'd be awesome, but thats been suggested a load of times so shouldn't be discussed in this thread.
[Edit] Found a couple of things on mircscripts.org:http://www.mircscripts.org/comments.php?cid=2161http://www.mircscripts.org/comments.php?cid=2252