This is for UltraISO 22.214.171.1242
The "Hidden" attribute doesn't seem to work if a UDF filesystem is used, so turn off the UDF filesystem in Properties for the image.
Then you use the File Properties dialog on the files/folders you want to hide and select the Hidden attribute (using Hide/Unhide is not the same thing - it's the same as Advanced Hidden).
And save the modified ISO image.
That works for me.