I will be writting a DLL that does this (after I finish the DLL I'm on right now - sometime in July)... Do don't expect a release overnight. Maybe by the end of this year... Watch my web site for a release.
If anyone wants to help write, let me know...

(Hooks and Subclassing experience prefered)
Currently planned features include...
* Auto Indent
* Color changes for comments
* Tooltips that follow the typing of identifers in progress of being type
* Tabs that are placed in the richtext editbox, rather that tabing focus. (I think mIRC can handle tabs for indenting scripts??)
* Reassigning mIRC Hotkeys, so alt-a can be restored
* Group management (maybe a menu for groups?)
* Hash table editor
* A DLL method of locking-out the script editor (to prevent your scripts from being modified by it's users)
* A Dialog Designer
* Reorganizing the view menu to allow grouping of script in to sub-menus. Then additionally add more submenus to goto specific aliases