And if you need something while waiting for the modify_lead api call to change, you should realize that this is easily modified with straight mysql from any sql client. If you have a "system" making the request, that system could instead run the sql command directly. Depends a bit on the trust level of the system placing the api call.
But I do agree that any field should be updatable via api. Just that until someone pays for that ... it likely won't exist!