G'day markubig!
I cannot say much about the Omnibounce but have you ever considered purchasing http://www.lumiquest.com/
Pro Max system http://www.wrotniak.net/photo/lq/index.html?
However, what do you do in the case where you have walls and ceilings that aren't white (i.e. wallpaper, wood-paneling, different-colored paints, etc.)?
Well in difficult lighting conditions like above you probably can do the following:
1)Use a gray card held next to the subject and use custom WB measured off the gray card. That should hopefully correct some of the colour cast.
2)Get a white/grey/black card like one you can find at the back of S. Kelby books and get the subject to hold it in the first shot then use the values to adjust your black/white/neutral colours via curves or levels.
3)Shoot in RAW. This allows you to capture maximum available information and then use CS RAW Import to adjust for lor casts or underexposure.
Additionally, how do you use the Stofen Omnibounce in dark rooms with high ceilings (such as party halls)? Should you be pointing it at the subject in this case? and if they are roughly 10+ feet away, should u just remove the omnibounce from the flast? I'm asking this question because i am getting frustrated with it underexposing and I have the FEC already at +1.5.
You can try the following if Omnibounce is the only thing you have. Ideally you would probably need something like the Pro Max 80/20 system with white insert if the ceiling are over 2-3m in height. But try the following:
1) Point at the subject and dial FEC down by a full stop to see if you can reduce those flash lit area by direct flash. The problem is of course that 420EX is fully automatic flash. Hence it will adjust flash output accordingly to produce "correct" exposure.
2)Consider purchasing off camera flash sync cord and holding the flash above the camera and to the side of subject to produce a different flash ilumination.
or maybe think about getting a bigger diffuser like a mini-softbox as it will allow you to use more of a direct flash and diffuse it over a greater area 
Well thats all I could come up with. Probably not much help I know but you are asking quite a lot from you camera (low-light situations, no surface to difuse the flash from, color casts from walls and others). At the end you may need to compromise with some of the settings to get the shot you need. My advice shoot in RAW then use raw converter to recover shadows or highlights as per this tutorial http://www.adobe.com …fs/highlight_recovery.pdf
Goodluck,
Flagpole
Sydney, Australia
