Ok, interesting response via email from Port Valet. When I asked about Norwegian overall, they didn't answer and asked me what ship I was sailing on. When I said Encore, they said yes the service was available and I could sign up. I then asked about Bliss (yes), Sun (no) and Jewel (no). No indication as to why some but not others, but they did apologize for the inconvenience.
Never sailed those ships or out of Seattle, anyone have a thought on why?