You can be sure that Genesis did Latin at Charterhouse and when Peter Gabriel commands the Hogweed army in the plural ("Kill them with your hogweed hairs, heracleum mantigazziani!") he obviously needs to pluralize it somehow, and so that is what he plumped for. Actually if "mantegazzianum" was 2nd declension neuter, the vocative plural would be "mantegazziana" - but that wouldn't sound so good. And anyways, as you note, Latin loan words get pluralised in non-Latin ways in English. In this case, he chose "-i". Get in!"