I believe PCs can use much longer pathnames than Macs.

Until Windows 98, PCs true filenames were "8.3" -- eight characters plus a 3 character file type (like ".doc"). Meanwhile, Macs (pre Mac OS X) had always allowed 31 characters -- pretty generous for a personal computer in 1984. Windows 95 added hackery on top of the DOS FAT file system to pretend to have 255 character filenames; but because they all were really 8.3 names with a mapping table, and didn't work on still common Win 3.1 systems, they weren't widely accepted until Windows 98 provided native long filename support with FAT32. Mac OS X, which is UNIX, also supports 255 character filenames. But even on Mac OS X, Poser 4 (a "Classic" application) was subject to the old filesystem rules. Poser 5 and Poser 6 shouldn't be subject to the old limit; and if they are, it's entirely their own fault. (Just as if they supported only the old 8.3 format on Windows.)

It would appear that Poser simply isn't allowing enough space to display long file paths in this dialog. That's odd, since I know I've had missing textures and the desired name was quite clearly readable.