I am please to report that in my diocese, the DoM of the Cathedral Church is also the DoM of the seminary (in fact he directs in two diocesan seminaries, not to mention a college choir.)
The seminarians are taught to sing a variety of 3-voice motets, learn a basic repertoire of Gregorian Chant, a mixture of traditional and some more modern hymns (hymns, not "worship songs") and are also expected to regularly serve at the altar in the Cathedral Church.
I think that realistically, the only way that seminarians can receive a proper liturgical formation is to participate in the liturgy of the Cathedral church and to themselves have received at least basic instruction in reading music, particularly in Gregorian Chant.
I have had the benefit of making friendships with a number of seminarians, a number of whom have since been ordained and are now assistant priests in various parishes. Hopefully this bit of "networking" will pay off later and land me a job. The latest groups of young priests being ordained very well appreciate a high standard of liturgical music.
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