I suppose I'll be able to contribute to this topic

. I can confidently claim I own almost everything conducted by
Boulez that has been (oficially or semi-officially) transferred to CD.

Well, of the Kairos discs you mention,
jessop, only two (the
Cerha and the
Neuwirth) feature Boulez as conductor. The third one (
Marino Formenti's album entitled "Kúrtag's Ghosts") includes some of the piano
Notations in the album's curious but imaginative and fascinating program.
Some of Boulez's early recordings are quite difficult to get on CD. For instance, he recorded
Mozart's first four piano concertos (or pastiches, rather) with
Yvonne Loriod for the Véga label. This has been transferred to CD by the CLASSICA magazine in France, and is available from their website:

More (live, in this instance) Mozart piano concertos were issued by Altus in Japan, in that series you've already pointed out:

I undertsand these Mozart recordings emanate from Loriod's perfoming
all the concertos in Paris within just one week in the early 60s (with Boulez and other conductors).
Also, there's
C.P.E Bach concertos for flute (with
Jean-Pierre Rampal) and cello (with
Robert Bax), recorded for Harmonia Mundi, and reissued fleetingly by them on CD in the 90s IIRC:

Then there's
Handel's
Water Music. The NYPO recording has had some circulation, and is included in the big Sony box, of course, but there was an earlier recording from The Hague, available on LP on Nonesuch in the US, and reissued on CD in Japan by Denon:

One thing that was never transferred to CD and may have been
Boulez's first commercial recording ever (for Decca) is
Darius Milhaud's incidental music to
Paul Claudel's play
Le livre de Christophe Colomb (not to be confused with Milhaud's opera). This was done in the early 50s when Boulez was music director fo the Renaud-Barrault theatre company, and includes a lot of the spoken text. It's so obscure, I've not been able to locate an image of the original cover. It is available for download from several sources, though.
More to come...
