You have mentioned you are working on a GMT model and I assume you will still be using the Eterna with some type of modification for GMT function. Do you know if this will be what some refer to as a "true" GMT, where the GMT hand is not slaved to the 12 hour hand? In other words, when you switch time zones one can adjust hour hand independently (not moving minute hand and seconds not hacked) without the 24 hour GMT hand moving. This is much nicer for travel, but most GMT models don't work this way. Two that I am aware of are the Rolex GMT Master II and Omega SMP GMT. I believe the Omega modifies an ETA 2893 to obtain this function, but could be mistaken.
Thanks and sorry if I confused the issue.