Well like I said, right now there isn't a way short of writing a programmatic lexical parser (or using our dynamic language XML definitions) to syntax highlight text. However in the prototyping we're doing for our WPF version of SyntaxEditor, which will have a lot of enhancements we've been planning for quite a while, I do believe there will be a very low level way for you to indicate highlighted spans. Actually our development code there is using just such a mechanism at the moment since we haven't gotten to the parsing design yet. This is something that was requested before so we're trying to find a way to expose it to developers in a good, clean way.
Once we get past some core editing features for the SyntaxEditor for WPF version, we'll be moving onto the parsing design. Another thing we'll be considering at that point is being able to load Antlr files since that seems to be a common request from our customers.
While we are going to be prototyping these sorts of ideas out with our WPF version, after it is complete we will be going back to update our WinForms version to use the new common document/parsing library that will be shared with the WPF version and will be trying to also have it use some of the neat new UI concepts we've been working on.