Is there a NodeJS ‘passthrough’ stream?

Yeah. Actually, by that very name. 🙂

stream.PassThrough

It’s available with Node 0.10 and later as part of the Streams 2 update (mentioned at the end).

It’s also one of the few types from Streams that can be directly instantiated:

var pass = new stream.PassThrough();

And, it’s currently documented briefly under API for Stream Implementors (towards the bottom of the Steams ToC).