I believe this ruling fits this situation as well:

http://www.wizkidseventsystem.com/bb...php?f=3&t=3175

The ruling there says that you can't Possess a team base character with an entity. So I don't see any reason you would be able to pick a team base character as Justin's Sentinel.

I was just about to quote the same ruling.

Team members are part of your sideline, so they are not part of your force until they are placed on the map.

http://www.wizkidseventsystem.com/bb...ideline#p15482

So you wouldn't be allowed to choose any character on the sideline as Juston's Sentinel as they are not part of your force. See ruling on Welcome to the Legion as well as this has a similar "When you build your force.." timing.

http://www.wizkidseventsystem.com/bb...ideline#p12443