Maybe the easiest way to do this would be to allow people to start a particular dialog option from a script:
RunDialogOption (int topic, int option)
and have some mechanism to return to the script when the option finishes. With just this, the AGS programmer can create full-colour icons for dialog options. You'd have to do quite a bit of scripting to surround it, but at least you could use existing dialog scripts rather than have to code up lots of DisplaySpeech es.
What do you guys fink?
Actually yeah, this was requested by Chrille a while back, so I've now added it to the tracker.
http://www.agsforums.com/tracker.php?action=detail&id=69