It seems unlikely that the type of people who shepherd Wikipedia pages in their free time would fail to notice and immediately report that type of censorship. The proof would be unnecessary - even the implication that Wikipedia was tampering with the content of its pages would have a huge effect on its reputation.
This isn't an unusual dilemma in publishing - print or online. If for-profit companies can adequately maintain editorial integrity despite pressure from ad partners, I think an ideological non-profit like Wikipedia can manage.
