Currently, all the CRUD subroutines for Notices/Letters occur in letters.pl, when they should be handled in C4::Letters or a new module in the Koha namespace. I always wondered where the subs for Letters CRUD were...and just happened to stumble across them today. Perhaps Chris will be doing something with this with his Koha::Email module, but I don't know.
Closing this due to age and lack of a concrete solution
Isn't the solution to have a proper Koha namespace library for the letters?
This is not solved completely as there is still some SQL in letter.pl, but certainly much improved with Koha::Notice::Template.