Well, a couple of things to consider...
1) You have to plan what you are going to do to the standby before you start the upgrade. You've got to make sure changes are not shipped to the standby during the upgrade you would are going to mess things up. You effectively need to split standby off before starting the upgrade on the primary, or all the DDL and DML will be shipped to the standby.
2) You activate a ADG standby the same way you do any other.http://www.oracle-base.com/articles/11g ... p#failover
You will need to check the upgrade guide to check the suggested approach and the data guard manual.
Personally, I would not do what you are planning. I would use flashback database on the primary to flashback to a PIT before the backup started in the event of a problem.