Just my 2 cents:
I doubt that changing a byte in a hex editor is enough to get a real valid "Mastered for iTunes" AAC file **.
If you read this official PDF from Apple you will understand that the process to get a real approved AAC file requires much more work:
If you only want to "fake" the description in your player this byte-change may work for you.
** Note from the above PDF (April 2021):
Apple Digital Masters replaces Mastered for iTunes as the name of the program to reflect the fact that these audio advantages are available to our entire music catalog across the Apple ecosystem, whether streamed or downloaded. All Mastered for iTunes releases are now badged as Apple Digital Masters and the Mastered for iTunes software tools are still usable for Apple Digital Masters creation.
For those interested in the technical details and oppinions from certified Mastering Studios:
(available in German only): Apples »Mastered For iTunes«-Downloads in Theorie und Praxis | SOUND & RECORDING