There is a book, published by the RSCM edited by David Halls called 'Open thou our lips' which contains a large selection of upper voice only settings of music for evensong. Certainly the situation is certainly different to when I investigated unison settings about five years ago, where Bairstow in Eb, Dyson in c, Wood in Ab, Stanford in D and the Howells services was about the only music of any real quality.
Whilst it is a travesty what has happened at Llandaff, with it does come opportunity. Perhaps other composers could be convinced to write settings for upper voices, to support the music at Llandaff?