I wonder if a .NET control already exists out there that could handle this? A quick search turned up ScintillaNET which may do the job. A built in version to LabVIEW would be handy though. I'd throw JSON into the mix too.
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This idea asks for control of the formatting. The linked idea asks for HTML rendering. HTML rendering is not exactly the same, but if you can control the formatting, you could cause some strings (i.e. tags) to evaporate entirely and change the fonts of things between those tags (for example). To some degree, they are strongly related, albeit not exact duplicates.
I'm going to add my kudos to this idea, even though it seems fairly low priority -- I can see its utility to my personal code on some rare occasions. But as a workaround, you should be able to implement this idea as an XControl today. You don't have to wait for NI to implement this functionality.