GTM iRule to set different TTL for each RR

it sounds like the logic is to be implemented by GTM itself (pools and load balancing methods on the wideip and pool), not the iRule. Then the iRule would inspect the decided answer and modify its TTL based on which DC the answer is for... you probably could not tell from an iRule what LB method was used, but you could just set the default TTL to 60 seconds.