Cómo convertir varchar a XML en una consulta OUTER APPLY

Solo usa una OUTER APPLY primero en forzar un CAST :

SELECT t.userid, r.z.value('@FieldName', 'nvarchar(MAX)')
FROM @t t
OUTER APPLY (SELECT CAST(t.xml as xml) as realxml) s
OUTER APPLY s.realxml.nodes('//Property') as r(z)