Stillwater,
you're getting some helpful responses so far by Pards, but I'd be a bit cautious. The Henry
was used as a military weapon and was issued with sling hooks, so there is ample evidence that it was slung.
As far as the '73 Winchester, I will wait for our National Judge, Trap, as to a historical counterpart.
This does not go to say that for your circumstances it wouldn't be allowed, but your original question was asking about the historical evidence as far as NCOWS guidelines. That being said, look to the military usage of the '73 Winchester by foreign armies, i.e. Canada, where the '73 was issued in musket and rifle configuration to Irregular units, and
was slung I believe, for what may be an acceptable configuration to alter your rifle. Such sutlers as S & S Firearms (best for repro military parts of the era) offer many types of add-on sling hook combinations and I would choose a known military configuration rather than the contemporary buttstock cup-type sling, simply because there is no counterpart in the document record of usage for the '73. The overwhelming majority rode in a scabbard or were stored on a rack somewhere. And remember, some types of leather hunting scabbards had slings. Perhaps another way to carry without having to alter your beloved '73 Winchester.
RCJ