I work for the same company as 747 and having seen the month of receipt gathering completed, there are a few observations that should be made.
Of the 10% to be selected, make sure that they are not Delsey Diners. It is imperative that those 'selected' fully use the daily allowance. If this means eating three square meals a day, then so be it. Some coffees for your colleagues can be bought- how about getting that pair of trousers dry cleaned whilst in a hotel? All essential stuff. Just get a receipt for it!
Those selected should ideally be people in other functions- union members, office workers etc, who find it easier to buy the requisite amount of food per day- it is no good if your roster keeps you in the air from morning to night. Preferably select someone well known for buying a round- we have such a well known Scottish gentleman who is very unlike his usual more frugal countrymen! I would imagine his bill would have been astronomical, which is what you need to offset those who are less able to spend.
Finally, select those who are able to afford to do the trial- it is imperative it does not fail as this has serious implications for the whole industry... 25% more tax on your allowances? How much will that be? Some schemes were set up for our company, to donate a tenner to those who were doing the recording or to leave receipts in a pigeon hole so they could be distributed. Make sure you book a Weightwatchers course for the completion. Good luck.