Wish I had someone to carry / lug the 600mm f/4 around for me! ha
If you really want to bring a 600 mm TC and you can handhold it for shooting, I think carrying it around may be manageable.
I bought the 400 mm TC lens last week. At 104 ounces, it’s a bit lighter than the 600 mm TC (115 ounces). To get used to the weight of the 400 mm TC, I’ve used it with my Z9 and added the 2x TC (9.6 ounces). Adding the 2x TC makes the lens combo weigh 113.6, or just 1.4 ounces less than the 600 mm TC.
I’ve taken the lens with Z9 and 2x TC out for several 1 1/2 hour walks. I’ve held the lens with one arm, and no other support, while walking. So far, that leaves me with a bit of soreness in the arm; but getting better.
On an actual photo trip, my plan is to use a Black Rapid strap to support the weight of the camera and lens when I am walking but not shooting it. I’ll generally have a hand under the lens to keep it from swinging (and as a failsafe). Black Rapid indicates that its strap system can hold pro cameras with heavy exotics. In a vehicle or in a zodiac, I may not have to carry the weight of the lens.
If you are walking to a place without shooting along the way, you might also carry it in a photo backpack. Not sure what the situation is in Haines. Maybe have a light zoom like the Z 100-400 out for the walk.
I have found I can shoot this combo (400 mm TC + 2x TC + Z9) handheld without too much trouble. My previous heaviest lens is the Z 800 mm PF. And on a trip, my setup will be a bit lighter than what I have been practicing with. I will normally not have the 2x TC on the lens (although it is nice to have access to 800 mm when desirable). I can also drop a pound of weight by using my Z8 body rather than a Z9.
One concern is holding the lens in shooting position waiting for action with no support. It may be hard to do this for long periods. I may bring a monopod to help with this. And on my next trip, we will do some shooting from a blind/hide. Here I may bring a tripod and Flexshooter Pro head to hold the camera and lens.
That said, I also think the Z 600 mm PF and Z 100-400 mm lenses would make a great, lightweight kit for this trip. I’d throw in a Z 1.4x TC.
Good luck. Sounds like a great trip.