Market researchers certainly can work on the aircraft in an official/overt capacity, or at least have done so in the past; I've seen them handing out and collecting (god awful conjoint) questionnaires.

The GPM (is that the correct acronym?) on-board questionnaire is now sealed on all sides, and has been for ages, but I've seen a CSM holding it up to the light before to read the comments. Clever. Conversely I was rightly complimentary to the crew on a flight to Cairo a couple of years ago, and was upgraded by the CSM on the return flight - she'd obviously read what I had to say!

Learned at the GGL drinks that BA is now trialling iPads for CSMs, which I believe include the chance to complete an electronic comments form which cannot be read by the crew and will be automatically sent to Waterworld.