IIRC when the Tristar entered RAF service in the 80s, there was an issue with cabin aie being too dry. Initially the aircraft had had humidifiers fitted; however, the original owners BA found these unnecessary and removed them to save weight. 300 pax provided enough humidity simply by breathing.
However, when the same aircraft was flying 10 hour sectors with only 3 or 4 onboard (I don't know how many crew the RAF used on them) the air was a lot drier.
The only relevance of my post is this: yes, some airliners have in the past had cabin humidifiers, but as to whether Boeing ever fitted them I have no idea.