First, the reason that LOAs were first sent out was to combat Early Action/Early Decision programs. B/c USNA has to rely on the nom process, USNA doesn't know whether it can offer an appointment to most applicants (those w/o Pres noms) until it receives the MOC slates at the end of January.
Civilian colleges were offering slots to well-qualified applicants in December and candidates didn't want to give up a "sure thing" to take a chance on USNA. Thus, the LOA was the best USNA could do to assure those well-qualified candidates that they'd receive an appointment. Not a guarantee, of course, but as close as they could come given the legal requirements.
USNA doesn't even consider college applicants until after first semester grades are in. Given that, for most college students, that's January or February, it doesn't make any sense to provide an LOA. Also, USNA isn't competing with civilian early decision programs with these students.
Thus, while I suppose it's theoretically possible, I doubt it would happen and I've never seen it.