Discussion about this/related to this at http://moodle.org/mod/forum/discuss.php?d=120541.
Could we have a 'scale' in Moodle that isn't really a defined scale, but rather one that maps to the letters and percentage ranges that are defined in the letter settings in the gradebook? In other words, if letters are enabled and defined, either site-wide or on the course, teachers would have an option of 'Scale: Grade Letter' in the Grade dropdown of any graded activity. This wouldn't be defined in mdl_scale, but rather from mdl_grade_letters. Moodle would simply know that this particular scale gets its values from the grade letter values, and would allow teachers to select the letter as the grade for an activity, rather than using a number that Moodle would then display as a letter.