The mIRC editor is cute, but it's not really suitable if you really want to write huge scripts. Why not just use some other editor? EditPlus has syntax hilighting for mIRCscript, and I'm sure you can let other editors do the same.
On top of that they allow you to use macros, change the indent size, autoindent, have multiple documents in tabs, etc.
For now, imho, the mIRC editor is unusable, mostly because I can't change it to show white text on black background, and also screws up my indenting (I tend to use 4 spaces, but as soon as I edit something with the mIRC editor, it immediately converts everything to 2 spaces, which is really annoying to put it mildly).
I also recall there being an actual stand-alone editor which had support for multiple documents, syntax hilighting, and it even had a fancy sidebar that listed all events/aliases in the current document. The downsides were that you could place your cursor anywhere, so if you clicked somewhere, that's where your cursor ended up, rather than at the actual end of the line as you'd expect, and the fact that you couldn't change something in the syntax hilight colours (afaik it was 'normal text' that was always forced to black). Can't recall what the name of this editor is, but I'm sure a bit of googling will reveal that.