Hello
I've understood that exiftoolgui may no longer be supported by it's author, but just thought I'd post anyway.
Using exitoolgui and the "ExifTool direct" feature I can do this:
-xmp:Custom1="Ĩ"
and my utf-8 char will display correctly in the workspace like this Ĩ.
If instead I select and paste the utf-8 char Ĩ into the workspace's editable field for Custom1 and save, the char now displays incorrectly: I