While I can use the portrait bar to display a class icon, it's not very useful that way unless it's on the left or right side, in which case it's huge and takes up a lot of screen real estate.
I'd like to request a module that adds an icon for class, similar to the many other icon options available.
Update: I've never done anything in LUA before, but I've managed to create a bastardized version between the Portrait module and the RaidTargetIcon module. I've actually got class icons to appear on frames now, but I'm running into some difficulties registering any options with PitBull except "Enable" so I'm not able to position or size the icons yet, other than a default position and size I specify. Also, they don't update when the unit in a frame changes (for example, when switching targets), only if the frame is cleared and recreated.
I'll pursue this further in the developers' forum.
Update again: It looks like several important features related to icons are hardcoded into the base PitBull and MetaLayout code itself. Adding new kinds of icons is not as simple as creating a new addon and registering it with PitBull. Since I'm not experienced in using the SVN, I don't want to potentially destroy PitBull for everyone by making changes.
I'll just restate my request to have the author add an icon module for class icons. Thanks! :)