IHG desk just contact individual hotel on your behalf. Just as good (maybe better) to ring hotel direct yourself. Has worked for me the odd time I needed to.

o The non-IC guarantee for Platinums is not as good as it initially sounds
a)Hotel can say anything is a special event
b)Special event is upto individual hotel/franchise and does not need to be logged in advance (or ever) with IHG
c)THEN when Platinum requests a room, hotel can
c1i)offer any room type it wants, including most expendive suite
c2)Price offered room any way it like, normalish price or skyhigh never charged rack rate or 3-400% rate rooms going for during event, think about expensive room at 3x usual rate when you want a basic room.

o IC brand guarantee for RA's is a bit better, as hotel needs book/clear event blackouts with IHG, and even then rooms often need to be made available except in exceptional circumstances.
IC guarantee is for RAs only, not for ambassadors or platinums.