I have noticed (and mentioned) that open()ing the root directory fails on some cards - in this case the fw opendir routine will be used as fallback. I have no idea why it fails. For me, it seems to happen on small (FAT12 or 16?) cards.
Thanks, I missed or forgot that. That could explain why they behave the same, still not sure why the neither long nor short would be listed when open and stat work

Summarizing the known limits so far:
VxWorks (A540 FAT16)
informational only, no CHDK LFN supportfilename 18 total 255 (>255 crashes)
DryOS r20
?DryOS r23 (sx100 FAT16,ixus80)
filename 18, total 255
DryOS r31 (D10 FAT16)
filename 18 total 59 (>59 crashes)
DryOS r39 (ixus120)
filename 30 total 32
DryOS r43
?DryOS r45 (g12)
filename >33, total 59
DryOS r47 (a3200, ixus115)
filename 31, total 59
sx220 is also r47 and behaves differently?filename >33, total 59
DryOS >= r49 (sx160 (r49) a3400 (r50), elph130 (r51) sx280 (r52), ixus150 (r54) )
filename 255 total unknown but > 500