Is it possible to request a single document by its id by querying an alias, provided that all keys across all indices in the alias are unique (it is an external guarantee)?
Yes, querying an alias spanning over multiple indices work the same way as querying one indice.
Just do this query over the alias:
POST my_alias_name/_search
{
"filter":{
"term":{"_id": "AUwNrOZsm6BwwrmnodbW"}
}
}
EDIT: GET operations are not real searches and can't be done on aliases spanning over multiple indexes. So the following query is in fact no permitted:
GET my_alias_name/my_type/AUwNrOZsm6BwwrmnodbW