When I inspected original keyboard electronics I saw that 8 rows of output pins are connected to Vcc trough 10k resistors. That means that the keyboard is pull-up configured? I was, also, puzzled by columns being connected both to first IC and pass trough to second MC until I read an excellent article about using a latch IC: latch multiplexing of switches or buttons | Open Music Labs . That made all much more clear.
I've got back to my arduino project and made small progress after connected pair of input pins to ground at the same time. There was an signal to midi yet random one. (bringing them from floating to zero?). Does that mean that the project code is meant for pull down configuration and must be changed?
Also, only way to get a response over keyboard is to reverse column/rows. I didn't noticed that before cuz the code is written for velocity only, meaning it must be both key switches pressed at short time to get an output. So how to reverse when "input" has single pin per key and "output" is in pairs? Well I used a trick. I connected to two different keys pressing them at the same time. That makes keyboard to finally give some signal, but same random data appears. Even when I put resistors to either pull-down or pull up configuration. Altho, the pull-up configuration gives much more stable result. Data is less random, I should say, but still not as should be.