I am currently testing Patch 6 also.
This is what I do to test from the Evaluation branch.
1. Create a new agent general policy by duplicating one you already have.
2. Name this policy - Update Virus Scan patch from Evaluation Branch.
3. Modify this policy and go to the updates tab under the policy.
4. Change the branch for the patch to say "Evaluation".
5. Save this policy.
6. Assign this policy to system you want to test the patch on.
7. You also need to make sure that any of your Update client task has the check box under Patches & Service packs for Virus Scan 8.8i checked.
8. Send a wake-up call to the system and it should install.
This is what I do and it works every time.
Create a new tag "Patch 6 test" and apply the policy to that tag and it's easy to test across a range of PCs.
I totally forgot about the method of using tag to test patch 6. I have just created a tag called Patch6Test and created a new policy assignment rule to use my policy that pulls patch 6 from eval branch. Works like a charm.