Great idea - not sure why I did not think of this!
My feeling is that it is not necessary to limit to large groups but can be expanded to other segments e.g. long stay guests, repeaters etc.
It should be fairly straightforward for the shop to liaise with the Front Office team on a daily basis to check the make up of daily arrivals and guests in house.
They can determine who should be allocated the vouchers and then the Front Office team can ensure that they are given on arrival.
For your budgeting purposes:
Group Segment for 2017
5,970 room nights in total.
Generally double occupancy - so allow for 11,940 guests.
These numbers are fairly evenly averaged throughout each month with corporates coming down from BKK at weekends.
The big picture for 2017 is:
Occupancy: 60%
Number of room nights: 42,940
Number of guests: 96,375
Not sure if this helps - but let me know if you need more detail.
Copying relevent members of the on-site teams.