Maybe there's something wrong about how your ISO is created. You mentioned you did use Nero 126.96.36.199 to create the ISO (either .nrg or .iso), using DVD-Video mode, but somehow Imgburn still cannot change the volume label in the UDF entry. Do I get this right?
Well, I don't have your version but 188.8.131.52, so I decided to rerun the test. I choose DVD-Video mode; under ISO tab, file name length is Max of 31 chars (Level 2), character set is ISO 9660, no relax restrictions checked; under Label tab, I enter 32 chars (upper/lowercase letters and underscores). However, only 15 characters are displayed next to the CD icon (a long time bug), that contains the AUDIO_TS and VIDEO_TS folders.
Mounted the iso, and only 15 chars are displayed as the volume label in the virtual drive. So I unmount it and use Imgburn to change the volume label. The created image has 2 filesystems: ISO9660 (containing 32 uppercase characters), and UDF (containing 15 characters that you see in Windows Explorer). So I fill up the UDF field up to 32 chars. Hit Apply and OK. Mount it again and the volume label is seen updated. You should see the image file has the Date Modified changed as well. So you see, Imgburn is doing its job.
You did not have it mounted while changing the volume label, did you? Make sure there's no lock to the file.
To check further, if you have the mkisofs package, use isoinfo program to check the various fields, it will only list the Volume id of the ISO9660 format. But it also say "BAD RRVERSION (0)" as it detects there's some flaw in the image creation by Nero. If you use another program, isovfy, it will list a bunch of errors, something like "RRlen=6 [BAD SUSP 0 0]". You will not see any of these errors for Imgburn created ISOs.
Maybe you should bring this attention to Nero support. Not the first line of support, but the last line of support straight to the developer's attention.