Be open about it of course, and send them an email as soon as you know who the money is going to.
Also give people the option of getting an email when there is someone in need. (To avoid annoying people perhaps limit the email to the next 3 people, then stop sending them for a user specified time.)
I imagine they would have an incredible amount of learning from the past several years that they would be willing to share. A lot of this has to do with perfecting the user experience for potential lenders / donors. It seems like you're doing a good job on this front but no doubt there's room for improvement.
I've been a loyal Kiva lender now for years and watched them grow to $375M in loans. I think you're on to something similar with tremendous potential.
Members of the Watsi team work for Kiva too (at least as of several months ago): http://news.ycombinator.com/item?id=4424745
Of course, the nice thing about Kiva is that it is a loan, rather than a donation, which allows you to use the same initial investment to help lots of people again and again.