Another problem is that lilypond is slow, and any WYSIWYG interface to it would either bypass the compiler to show a "preview" of the score or would have a horr

Another problem is that lilypond is slow, and any WYSIWYG interface to it would either bypass the compiler to show a "preview" of the score or would have a horrible lag whenever the user edited something. This might not be obvious, but adding or removing (or even changing the pitch/duration of) a single note can potentially lead to a solution with completely different appearance (after all it's a dynamic programming algorithm that lays things out, considering all sorts of "badness" in the style of tex). If you remove this specificity for some greedy techniques, then it will no longer have lilypond's quality.