This is an interesting thread!
I wonder if the class someone sneaks in to makes a difference to whether or not people find it acceptable / unacceptable? Is it OK and a bit cheeky to go from WT to WTP, but for the same person to go to CW or F would be taking the proverbial? Does it make a difference if you pay for your flights or not?
Would you change your mind if you'd set your heart on a particular menu item, and the last plate of it went to someone you had seen sneak in to the cabin but hadn't been spotted by the crew?
Me personally, I don't think I'd feel as aggrieved in WTP as I would be in F. I pay a lot of money to fly First or Club World, and it's my money not a super-generous company travel policy, so I would be pretty peeved if it became something that people just did because they knew they could get away with it, therefore I would flag to the crew if I saw someone do it.