We have a need that we like for the GTM to respond back with a NAT address rather than the real address it is monitoring. The systems that need access to the resource actually hit a local NAT that is then translated to the real address. At this point the failover is manual as the no DNS is currenlty used, but we would like for the resolution to come from the GTM to allow for automatic failover, so respond back with the proper NAT address due to monitoring the real address for availability.
You can certainly use iRules for this. Please see the following: https://clouddocs.f5.com/api/irules/GTM-iRule-with-NAT-translation.html. This includes also logic for checking LDNS, but you can strip most of that. Maintain the real IP to NAT mapping in a data group and reference this in the iRule. One approach that should work.
