The false assumption on your interpretation on high and low level is indeed mine.
Although such low-level control would be great to have i highly doubt its less hassle then route certain message loop events and commands from the script engine to winapi and other way around. Also adding support for a few more controls wouldnt even take that much trouble. The RichEdit as you quite rightly stated is a handful to port to a maintainable/feature rich implementation in mIRC, same why we at DCX feel the RichEdit should be redone completely as the first attempt to port exposed severe functionality drawbacks.
Some more events and controls and more control over the fonts/colours/icons will work miracles in useability and more importantly scriptability.
