In deciding whether or not to keep a promise made to someone who has died, I would ask the following question: Would that person have wanted you to keep the promise even after he or she died? Some promises are specifically about what one is to do after a death ("Promise me that you will send money to my daughter") while others are clearly irrelevant after a death ("Promise me that you will come to the beach this weekend"). With other promises ("Promise that you won't loan her any money"), the answer to this question is not so clear cut, but IF the promisee would have wanted you to keep the promise, that should count as a reason to keep the promise.
As you say, there are cases where the costs of keeping a promise outweigh one's obligation to keep that promise. These could be thought of as cases of competing duties (to uphold a promise versus to save a life, for example) or they could be thought of as qualifications implicit to the promise itself (as understood by both the promiser and the promisee). In neither case, though, should the death of the promisee change these calculations.
Breaking a promise to someone who has died does not need to be thought of as damaging that person -- though it may be possible to damage a person after death. Breaking a promise -- especially under circumstances in which there is no longer anyone to hold one to one's promises -- can also be thought of as damaging the larger social fabric that depends on trust .