I would have to agree with John. The first WW was notable not so much for the repression of anti war sentiment, the treatment of conscientiuos objectors is well documented, but was hardly different to the second. The main difference so far as I can tell from the reminisences of relatives and published sources was the lack of any focussed opposition to the war amongst the general population, indeed until the massacre of the Kitchener battalions on the Somme, it was quite the reverse. This feeling continued amongst the civilian population as evidenced by letters to the Times etc registering shock at the anti war ("Trench") songs sung by troops whilst marching, and persisted until late into the war if not to the end as demonstrated by Sassoon's "Blighters" which includes a reference to a tank. The likelihood of anyone risking both official censure and public opprobrium by publishing or attempting to publish, either as sheet music or recording, anything with an anti war sentiment is, in my opinion, vanishingly small.
Actually, I would disagree with your friends basic tenet, I suggest that the widespresd distribution and knowledge of the "Trench" songs then and now, is a perfect example of the folk process getting a message "out there" far better than any published source of the day could have done. But then this is a folk site, so I'm biased.

Pete M