Of course, once posted, I see a related topic 
Incluir imagens em massa em coleção - I translated the conversation and alas looks like I can use the special_attachments for the non-primary images. Key tip pasted below in English in case anybody else finds this post via English keywords;
"…special_item_id of each object, so that Tainacan understands that it is an update special_document with file: followed by the URL, which defines the main image special_attachments for the attachments; if there is more than one image, separate by || "
Kudos to @andre_benedito