Had this issue the other day and took me alot of searching around to find this solution, thought others might be looking for this too. it sounds easy enough but I never expected it to be in the place it was, could not find any KB's that had this either.
When you have checked in the new engine package into the evaluation branch.
Create and edit a new agent list - Got to the "Updates" tab and select "Evaluation" under the repsoitory branch to use for the engine update types.
Then just modify branches/individual workstations to use this new test list, wake up the agents and there you have it.
McAfee Administrator for QH
Good work around, I assume you're using the new agent 4.5 which has put this limitation of deploying using the "evaluation branch". I hope McAfee will fix this with Patch1.