I already have, and they pretty much gave me the option of hiding the GUI entirely or upgrading. I just hope the feedback received makes the decision makers behind these decisions re-think this next time.
In my experience with Pre-6.5 agents on XP machines and in some cases, Win7 machines, I've always had a problem getting the agents on these machines updated. Even with GPO, these agents would always fail to update. I would end up just copying the EsetUninstaller.exe to these machines and running them in safe mode to get rid of the agent and av software.
I too had spent a lot of time fiddling with ESMC and GPOs to get the Agents updated but gave up and did it the hard way.
That said, have you taken a look at the agent logs on the problematic machines? These logs should at least point you to what's bugging the update process. That's the way I had figured the XP agents weren't liking the update command.