What’s the best way of reporting this issue directly to Apple so that the driver angle can be looked at?
It would be best for OmniGroup to report the issue to Apple through their developer channels. Omnigroup can work directly with Apple developer support to identify the cause and determine a solution.
If you want, though, you can report the issue to Apple through their feedback pages:
http://www.apple.com/feedback/
but the issue will get more attention if Omnigroup reports it themselves.
For what it's worth, I have OG 5.1.1 on an iMac with the ATI 2600 and dual displays and have not noticed any graphic corruption, though I have used OG only very lightly. Not to denigrate Omnigroup's tech support, but perhaps if you can describe the problem in more detail - how it appears, if it seems to be related to any particular type, size or content of document, etc. - someone here might have an idea.
Regards.