"Latn" either in the registry (machine-readable part, not only in a comment or description), or in the tag. "In the tag" is dubious for various reasons, e.g., it is massively redundant, there should be only one place to publish the "pinyin is Latn" info. OTOH "in the registry" cannot work for generic variants such as "fonipa", unless 4646bis allows something like "*-Latn" in prefixes. Frank