I think it's a bit risky to pay for it in ISK if you don't go through the official method.
Actually, over the past year or so I've bought TCs for a few people in Deii Feram and Bat Country who didn't have credit cards and couldn't get at their ISK to repay me until they re-upped.
On the other hand I kicked someone out of DF for asking me to do that. I like to keep em guessing.