A before script would definitely work, however I would suggest editing the field map with a script because that way you have all the logic together at the field map level. Open your existing field map for the true false field and then check the Use source script box and paste in the following:
if (source.field == "t" || source.field == "1") {
answer = true;
if (source.field == "f" || source.field == "0") {
answer = false;
These scripts need to have answer="ANSWER", but otherwise they are like any other scripts.
