I'm pretty sure they could do the "To Serve Man" situation openly with a sign-up sheet. Giving humans good final days would be easy for them. Fixing medical issues, enrichment, no stress, get to see space etc.

It could even go along with the brain roomba thing I mentioned (or something else like a VR network of brains) as I assume they probably wouldn't want to eat brains anyways.

Well, that is until the gov't stepped in for one reason or another. Give people a choice like that and it'd probably be noticeable for the population even if the aliens gave a wide window for people to change their mind. Society would really need to step up, probably an impossible task even if humanity was given advanced medical technology.