Which instrument for arbitrary signal commutation (matrix)
If you can get away with analogue switches you should but, if you can't I've designed pretty much what you might need and used pickering reed relays (type 107): -



Who cares about reinventing the wheel - wheels work so use 'em.

You can stack'em up close - I had about 96 differential input channels that needed testing on a pice of equipment - we used the relays for sequencing thru and injecting signals. They are good for very low level signals (wetting current problems etc..). And, they do a changeover version I've used on a later bit of equipment.

They also (and here's the purchasing recommendation!) do modules that are addressable.