IC *easter eggs* hacking & keyboard matrix

[wax+wire]

I'm bending my Casio MT45 and it is going well so far (well, I made it work fully which is good)

this site: http://weltenschule.de/TableHooters/Casio_CT-410V.html talks of easter eggs, or unutilized potential in circuits ICs.  How do you go about accessing these features?

The MT45 uses to main ICs

NEC D930G 011 - for accompaniment

HD44140 - for the main voice

As a start I'd like to separate the accompaniment out into drums, bass, arpeggio and then i can have volume control over each part/cut bits.

Thoughts?