I thought ROS 2 added a way to set a parameter “in a namespace” that affects all nodes in the namespace (during launch time). But I’m not sure when/which version.
Also, not all nodes necessarily need to share the same clock. You can have some nodes that need no clock at all. In ROS 1 it was the nodelet load node that unncecessarily spent large % CPU just decoding /clock messages which got thrown away. Or you can have some other realtime monitoring/HITL node that just needs to run according to the real time.
But all in all, I share your request. It would be great to have a centrally-configured sim time. What about environment variables?