BMSONで文字列をescapeする
"TITLE": "𠮷野家",
U+20BB7をescapeしたい。\u{20BB7}
”
To escape an extended character that is not in the Basic Multilingual Plane, the character is represented as a twelve-character sequence, encoding the UTF-16 surrogate pair. So, for example, a string containing only the G clef character (U+1D11E) may be represented as "\uD834\uDD1E".
U+10000
の文字をescapeするなら、
"TITLE": "\uD842\uDFB7\u91CE\u5BB6",
BmsONEは文字列の4桁escapeを正しく読み取り、U+10000
に対応していないように見える(VL Gothic自体はU+20BB7を表示可能な書体なので)。