[sdiy] first-of-n or last-of-n circuits?

[Electronic Battle]

Hello

My old Korg PS-3100 polyphonic synthesiser has a multiway connector on the 
rear port; I remember looking at the circuit diagram and realising that a 
suitable switch-output keyboard could drive it (can't remember the details 
but there are 48 pins relating to 48 keys - when you take a pin high or low, 
the note sounds).

If I could develop a simple circuit (74HC or similar family i.e. NOT using 
any form of PLD) to interface the keyboard, I could switch between normal 
polyphonic control, or I could set it so that the last of-n-notes played 
souds, over-riding earlier ones. High note or low note priority, that sort 
of thing. Then I could play faster solos on it without messing up by hitting 
two keys at once. It also gives me the option to "double up" ; in other 
words, set monophonic mode to play two keys for each keypress, but those 
keys are an octave apart.

I'm pretty rusty so can anyone give me a hint at the sort of mulitplexer or 
other (address decoder?) type of IC I might want to look at?  It 
instinctively sounds like a simple problem to solve but I can't see how at 
the moment.I am unable to program anything so I would need to solve it using 
glue logic.

Thanks for any pointers

EB