Interesting...I don't see why it wouldn't be possible. For one, you would need to think about how players can understand what kind of buff they provide (if it's meant to be specific to the banner you are picking up). For two, the difficulty of introducing "destruction" depends on what you have in mind exactly (f.e. you could just block other troops from picking it up or remove it from the mission when it drops).
For one, yes ideally it would be nice to show the current effect of the banner and apply it once the player grab it.
For two I was more thinking about having the possibility to somehow interact with the banner and "destroy" it, then apply the effect.
Anyway, that's just some ideas I got while watching the video, I can't wait to test them in their "native" form