Note that the 50% off vouchers are officially only good for the rack rate. You will not be able to get the JW Bangkok THB 8.000 rate down to THB 4.000 using those vouchers. If staying at the JW Bangkok i usually use the "Preferential Fixed Rate" coupons. My 50% off vouchers are used for stays at the Singapore Marriott (quite hard get a discounted rate there) or the JW Phuket (they usually reduce the shown rate on marriott.com by 50%).

As for the creativ usage: Indeed one can be very creative. However i do hope that there are not too many card holders "overplaying" it and therefore forcing Marriott to "enhance" the programm.

Regarding the reservation: Mentioning rate codes "GGLA", "GGLP", "GGLQ" to the agent might help her/him to find the specific rates for Gold Card Members.