Catherine W. Reilly's wonderful 'Bibliography of English Poetry of the First World War' lists thousands of poets and their work. Among them is this entry on page 110:
"G. de St. Ouen 'Lest we forget': a note addressed to H.I.M. Wilhelm II, Emperor of Germany, on behalf of the British volunteers from the republic of the River Plate, Bueonos Aires. Published by British Printery in 1916."
Unfortunately, I cannot find a mention of G. de St. Ouen or of the British Volunteers from the Republic of the River Plate. If anyone has any information, please get in touch.
Reilly, Catherine W. ' English Poetry of the First World War A Bibliography' (St. Martin's Press, New York, 1978) - presumably published to mark the 60th anniversary of the end of WW1.