Evening all, a few more:
Samurai Showdown 4: Doesn't work, gives loading screen then dumps to NGCD console.
Fatal Fury Real Bout 2: Doesn't work, as above.
King Of Fighters 99: Doesn't work, goes to loading screen then hangs.
Over my few trials, I've detected 3 different failures: doesn't do anything (World Heroes 2), gives loading screen then hangs (KOF 99, though this may be the same as the previous, as I'm not sure WH2 has a loading screen; Metal Slug doesn't, after all), and gives loading screen then goes to NGCD console (SS4, FF RB 2 and Special). Possibly the last fault is due to some missing component of the boot sequence rather than low-level incompatibility; could someone who knows what they're talking about offer any hypotheses?
EDIT: while I'm here, Samurai Spirits RPG also goes to NG CD console menu. Not that I would have been able to play it anyway.