FWIW The regs do state the following:
Leave the head, tail, and all fins on your catch until you get them to your permanent (ordinary) residence.
In order to enforce quotas and size limits, our officers must be able to count, measure and identify your catch. ... Ensure your fish can be identified, counted and measured if necessary
I totally get where you're coming from and I do think it should be OK, but I guess it comes down to your comfort level as to how much you trust someone else's ability to ID a species from a partial carcass. I personally wouldnt press my luck, but you do you...
Regs state a retained coho must measure 25cms, so if the tail is intact but no head and it measures at least 25cms with no head I should be ok. Pretty easy to identify a hatchery coho.
Probably OK? but beware of the '2+2' systems where only two adults are permitted, and if there's any question as to whether something is a jack or not. If it's missing a head, it might appear mighty suspicious ...