Yes, connecting a Port to a line out on a receiver would work fine. In general, you can think of the Sonos Port as a “tape deck” that connects in the same way, i.e. to a tape loop (rec in/play out). It should never be connected to a Phono input, though.

Anything that you then send to the Port from the receiver would play in sync across all your Sonos speakers, assuming you’ve grouped them properly. And, if the other side (the out on the Port to the input on the receiver, the receiver would also play stuff, if you had other speakers connected to it. 

Important to note, however, that anything that you send to the Port (and play on your receiver’s speakers) will be delayed slightly on your Sonos. This isn’t an issue if you’re just using the Sonos speakers. If your receiver is getting the feed back from the Sonos, it will al be in sync, but if you try playing something locally and send it to the Sonos, you’ll hear that delay. 

Thanks Airgetiam, your quick response was helpful.

Don’t have the equipment to test this here.

What would happen if you hooked the Port to the tape-out and tape-in jacks, selected your source on the receiver but also selected the tape-monitor option.

Would the Port send what it saw on the input to the outputs and would it be the delayed signal or just a copy of the inputs.

So: Phono → Rx → Port → Rx