I gave bowsing UDF BD-Rs another try and noticed that you can indeed access all BD-R subfolders from the XMB, under Photo, Music and Video. (Unlike when using the multiMAN File Manager, where you can only access 2 levels)
So whatever the reason for it not working in multiMAN is, it appears to be unrelated to PS3's GameOS; otherwise it wouldn't be possible to browse data BD-Rs from the XMB.
Unfortunately there haven't been any updates regarding this since the first release of multiMAN. Don't you think you could give it another look? A working file manager would make multiMAN the most powerful PS3 utility.