Normblog has an interesting selection of articles up in a post today including analysis by one observer suggesting that Diaspora voting could actually be used by ZANU-PF to its own favor through fraud.
There are a number of international and other organizations equipped to assist in "out of country" or diaspora voting. IOM (International Organization for Migration) is the principal one that comes to mind. IOM has considerable experience in organising and managing external registration and voting and in support for the inclusion of refugee and displaced populations in the electoral processes. It has organized significant out-of country votes for Bosnia, Kosovo, East TImor, Afghanistan and Iraq. According to its web site:
IOM's out-of-area voting has cumulatively reached over 70 different countries, hundreds of thousands of verified nationals eligible to exercise their right to vote, in a combination of mail and in-person procedures in the largest external voting programmes to date.
Two other American organizations, IFES (International Foundation for Electoral Support) and IDEA (International Institute for Democracy and Electoral Assistance) also have expertise in the area of "out-of-country" voting.
So, with proper third party supervision, there is reason to believe that it would be possible to organize fair and verifiable diaspora voting--that would not be prone to manipulation by ZANU-PF. This would be especially important for the 1.5 million estimated eligible Zimbabwean voters residing in South Africa.