I'm not sure what an input router is, but the concept of mouse chording is absolutely an inherent property of Plan 9/9front and Acme. To the best of my knowledge you cannot mark, cut, copy or paste text at all using the keyboard. Copying is especially cumbersome and involves a three-button chord in conjunction with some precision pointing (first to mark the text, then to where you want it pasted).
I'm sure this could be implemented using keystrokes as well and the source code is of course available. However, I doubt such a change would be accepted upstream. You could say it's a central point in the "Plan 9 philosophy".
https://plan9.io/sys/doc/acme/acme.html