This is the matrix principle:
Last night, again in bed, I found the solution; in this matrix I can put the multiplexer in horizontal or vertical orientation. In the latter case it is possible to have one-to-many connections while the multiplexers only open one port at a time.
It would require 16 multiplexers though, making the project a bit more expensive, but still manageable,
BC
I guess it's what I was trying to tell you all the time 
Well, it's not a straightforward solution, as considerable complexity is added by the fact that that uptil 16 multiplexers need to be programmed to route to the outputs, whereas my (and probably yours) initial idea only one multiplexer per input had to be programmed, but this had the important disadvantage that only one output per input could be picked.
but your suggestion definitely put me on the right track,
BC
You might also consider using a Teensy, you're still working in the Arduino realm and it has excellent support. Here is a MIDI patchbay discussion in their forum and there are probably more: 8 X 8 MIDI interface patchbay
bobcorr:
You might also consider using a Teensy, you're still working in the Arduino realm and it has excellent support. Here is a MIDI patchbay discussion in their forum and there are probably more: 8 X 8 MIDI interface patchbay
Can't understand I didn't find this page in my lenghty Google searches, thank you for it. In my case the Teensy has no specific advantage over a regular Arduino, but there are some very interesting links on the page you refer to,
BC
There exist multiplexer and demultiplexer chips, which can select 1-of-8 by 3 bit addresses, or 1-of-16 by 4 bit addresses if you need more rows or columns.