The scripting language is tied into almost every feature in mIRC, including the GUI, so you could say that the script parser is literally mIRC. I am not sure how I would create an mSL script parser that is not mIRC. Yes, any non-mIRC mSL parser would need to have been reverse-engineered to try an act like an mSL parser. Considering that mSL grew organically, was not designed or planned, and has so many nuances, differences, backward-compatibilities, edge-cases, and inconsistencies in behaviour across pretty much every feature... well, let's just say it would be a challenge.
The only approach I can think of that comes close to a headless mSL would be to add a command line option that prevents the mIRC GUI from being displayed. But there are so many commands/identifiers that relate to dialogs/windows/other GUI states that even that would take some work.
