Okay, so I reordered the priorities because I was running into trouble having the highlighting automatically work from quick reply. Instead, I've now added a button to go to plain text and back to highlighted text. I suspect this probably needs to be a user setting, since having to toggle all the time would get pretty annoying. So the remaining items are now (in my new priority order):
1) automatically pick up the right kind of brush from the forum URL (right now it's always C++, but it should be possible to properly color C# and C)
2) Make it automatically run when a post is submitted. (For now, it falls back to the old code tag box until a page refresh. Note that this impacts only the poster--anyone loading the page will see the highlighting, as will the poster once refreshing.) My first attempt at fixing this failed; I need to look at the sequence of a "Post Quick Reply" operation.)
3) Adding a user setting for disabling syntax highlighting.