A good idea. For a start, we could ask the respective publishers directly for permission.
The Simplex may be the easier case. Once upon a time, I wrote to the Libreria Editrice Vaticana asking permission to copy a chant from the Graduale Simplex, to print it on T-shirts. They kindly waived any royalty fee, since I was proposing to donate any profits to a charitable cause.
(The project never came off, by the way.)
So I figure they are not very demanding about getting royalty payments!
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