FWIW, I tried this with Kodi and the libbluray/libaacs/libbdplus stack (with the patches from candela, of course) and wondered why BD+ wasn't working, despite the convtab.bin's being at the right place. I found out that after selecting the desired title from Kodi's title selection dialog, it always said "bdplus_m2ts(...): no conversation table" without any real traces of libbdplus doing anything beforehand.
So I patched bdplus.c:bdplus_m2ts() to simply trigger a BDPLUS_RUN_CONVTAB event based on the bdplus_test.c example and finally got it working that way.
What I did to "fix" this might be a very bad idea, and even libbluray or Kodi might be doing something wrong, but I thought anyway I'd share my findings and also the patch to apply to the libbdplus sources, see the attached file.
Maybe someone finds another way to fix this "the right way", though until then hopefully this is of any help for anyone.
Thanks to candela (and whoever is involved in this) for getting BD+ playback to work in this way!
EDIT: Seems like the attachment needs approval, until this is done, I've uploaded the patch to
https://anonfile.com/U0Hfh885nf/bdpl...vtab.patch_txt (no mega account, sorry).