Note that there is also an “overwrite” check box in the Write node which prevents is from overwriting frames when unchecked.
Just disable the write node if you really want to connect your viewer to it.
Note what you consider an issue with Natron comes with a feature: do you know a way to execute an external program somewhere in your graph in Nuke? AFAIK there is no way to do it. In Natron, just use RunScript.