Holtek Taiwan often mention Mask ROM options in their datasheets, I believe that was the source of the confusion you had. It's a nice thought the chip is OTP, but alas, no! Entirely differernt technology!
The information about Mask Rom layer appearing in the datasheet isn't meant for the EndUser. For instance when Holtek make a new production run of Melody Generator ICs, the Client can specify a custom melody of his own to be programmed into the Mask Layer used during chip fabrication.
It would be interesting to learn how much it would cost to customise a run of HT6221s.