This is what I see when embedding a subtitle in an ePub using calibre (the subtitle is "Daring Adventures on Elephant Island":
Code:
<meta name="calibre:user_metadata:#subtitle" content="{"table": "custom_column_9", "column": "value", "datatype": "text", "is_multiple": null, "kind": "field", "name": "Subtitle", "search_terms": ["#subtitle"], "label": "subtitle", "colnum": 9, "display": {"description": "", "use_decorations": 0}, "is_custom": true, "is_category": true, "link_column": "value", "category_sort": "value", "is_csp": false, "is_editable": true, "rec_index": 31, "#value#": "Daring Adventures on Elephant Island", "#extra#": null, "is_multiple2": {}}" />