Good idea. I see a MP Coop on top of a hill. Remember to take a screen shot when it's done
The solution seems simple to me. The empty flagpole wont display any flag at all, unless you use setObjectTexture to do so. It should be fairly easy to include the ending conditions in the same script, trigger, whatever that fires off the texture command.
I'd go for a mix of addAction - (script) - setObjecttexture & endMission
Your question is rather non-specific. Thus the non-specific answer to your post.