Need a a flag in the custom field list to allow custom fields to apply to multiple callsigns.

[darksidelemm]

Suggesting adding a parameter "other_calls" with a list argument of other callsigns the custom fields applies to.
e.g. in here https://github.com/projecthorus/horusdemodlib/blob/master/horusdemodlib/payloads.py#L208 if the other_calls list exists, add in entries for each of those callsigns.
Care will need to be taken not to overwrite existing entries (or at least raise a warning if this happens)