Hi,
I couldn't identify a tag for conductor in AAC files. When you fill in that field in iTunes I think it just saves the information in its own database.
If you think you can tag an AAC file with a conductor field, please do it and email me the file (telling me the name of the conductor you put in the field so that I can search the file for it). I will then investigate. Please send the file to:
p DOT sanders AT alpinesoft DOT co DOT uk
Thanks.