Tentatively good news.
I spoke to Uto.
He built his converter on the old ZX Next specs (with only 4 shades of blue).
I told him that you informed me that 9-bit colour is now available on the Next (max 256 out of 512 as you said).
It appears the export from adventuron doesn't require any changes (because I left palette allocation to the SC2DAAD utility which is part of DAAD Ready).
Anyway, Uto told me to say "tell him I found he is right, and I'm working on it".
Huge (huge) thank you to Uto for DAAD Ready / Maluva / SC2DAAD and the DRC compiler.
Fingers crossed.