I've known lots of people who have used incentives to get people to complete questionnaires/participate in studies (for example, questionnaires are all put into a raffle, and first drawn wins an MP3 player, 2nd prize wins x - etc, etc, etc ... or substitute the gadgets for cash prizes).
Remember ... if you look at most published studies they've also probably used some type of incentive to get people to participate, so if they can do it, than it shouldn't be a problem for 'mere' PhD students.