This is still at the theorical level, until one of the hardware gurus gets his hands dirty and tries to put it in practice.
The NMK004 cannot be dumped directly using the common methods, but the way it operates can probably be used to extract the internal ROM data in another way.
The external ROM contains data tables used to play the samples and music; but not only that, it contains pointers to those tables.
By changing those pointers, we can make the NMK004 use its internal ROM as if it was one of the external tables. In particular, this can be done with the table that contains the sample numbers to be played by the two OKI6295.
We can then log the commands sent by the NMK004 to the OKI6295, and this will allow us to determine which sample it is attempting to play - and therefore the value of the internal ROM byte it has just read.
So in theory there shouldn't be problems - unfortunately the hardware setup needed to get it all to work isn't trivial. But I'm confident that in due time it will be done.