If you look at the ITextRangeProvider::GetAttributeValue() documentation in MSDN, it should allow the user to specify an attribute they want, e.g. UIA_FontNameAttributeId, UIA_IsItalicAttributeId, and it will return the value. We would probably have to specify some custom attribute for things like Squiggle status, Squiggle color etc. Adornments may be UIObjects of their own and not exposed as TextRanges.
Since different users of Syntax Editor may have different requirements, we will need different attributes from GetAttributeValue and it will be more flexible if we could override the TextRangeProvider. Can you make the TextRangeProvider class public so that we can do our own implementations?