This reminded me very much of the great effort that Knuth (as a commenter mentioned) put into both his monumental TeX formatting system and MetaFont font genera

This reminded me very much of the great effort that Knuth (as a commenter mentioned) put into both his monumental TeX formatting system and MetaFont font generation program in a (imho) successful attempt to enable the preparation of beautiful math documents. One thing that made TeX so successful is that it was designed to be open, and many packages (e.g., LaTeX) have been written to made it easier to use and more powerful. If LilyPond can be easily extended in the same way (i.e., programed in LilyPond, not C++), it might be similarly influential.