And even if they define "Protestant" broadly enough to oinclude Calvinists, Wesleyans, and Baptists of all stripes, it would not include the Plymouth Brethren, who do not see themselves as Protestant (despite having a quite Protestant theology) - even though they have quite a few students (and faculty, I believe) there who hail from Plymouth Brethren churches. (Yeah, I know. It's an obscure group - but one with which I am personally familiar.)
I bet they'd be readier to employ such a[n Eastern Orthodox] than a Catholic who says he can sign it.
I doubt if its come up. An evangelical meets Catholics all the time, but many are blissfully unaware of Eastern Orthodoxy - and it's not as though the Orthodox are beating down the doors to teach in Evangelical colleges.
Plymouth Brethren have always been very common and highly influential at Wheaton. That's why the doctrinal statement is silent about denominations and confessional traditions (Lutherans, Anglicans etc.). But the bulk of Wheaton's constituency has always been Bible Church, Free-Church, low-church rather than Lutheran or Methodist. Presbyterians were very dominant in the 1930s and 1940s, even the 1950s but that's because the Presbyterians split under Machen and the ones at Wheaton represented that classic Princeton theology Calvinism that had been booted out of PCUSA. But Presbyterians are, among the mainline denominations, the least "denominational" and least liturgical, so they were closer to the independent Bible church heart of Wheaton's old 1920s Fundamentalism than Lutherans or Methodists or Episcopalians would have been.
So Plymouth Brethren fit right in at Wheaton, indeed were a core constituency. I don't think you are quite right to say that PB's refuse to be called "Protestant." They do of course, but only because "Protestant" to them means "denominational" and they have come out from the idolatry of all the denominations and confessions to the pure congregationalism (the more honest among them recognize that in many ways PBs have become denominational). Certainly if you pose "Protestant" as meaning "non-Catholic" and "non-Orthodox" then they would by all means identify with that.