The enterprising Phil Trammell says in an FB message
"I just got off the phone with Vanguard Charitable, and they say that there's no issue with setting up an account from overseas. The catch is that contributions have to come from a US-based bank or investment account...! Since anyone can contribute to anyone's DAF, this doesn't seem too hard in principle; you can just contribute by sending money to a US-based friend, or that rethink charity donor-swap org or something, and they deposit it in the account you manage online. (It could always be done in small batches, if you don't fully trust them.) The initial contributions wouldn't be tax-deductible (or maybe they would be for the friend you're initially sending the money to?), but at any rate the interest still wouldn't be taxed.
Maybe this too much of a hassle for any individual to bother sorting out, but if there's interest from enough other people, and someone's willing to set some arrangement like this up, would you want to use it?"