I guess I would like to modify my suggestion to: "Don't ask for donations on gift certificates or don't allow donations to be taken for gift certificates unless explicitly permitted by the gift sender."
That would be great - but it probably won't work (or at least not without a quite sophisticated system). Why? Because you don't pay "with the Gift Certificate" but with Kiva credit. This Kiva credit is one big pool of deposits, repayments, refunds, and redeemed GCs. So Kiva would have to track where the credits for each transaction come from (using a certain order of charging) and then adjust the donation amount accordingly... and explain that to the lender - or Kiva would have to keep the GC amount separate and ask the lender whether he wants to use the GC credit for this transaction or not.
That's why I settled for my "include the donation in the GC" suggestion which is not ideal but would be something.
Best wishes,
Wolfgang.