I'm not sure I understand your issue, but it seems like you want to construct a json string by hand without the root object braces.
Here's a quick example:
struct Row {
string value1;
string value2;
}
Row[] rows;
rows ~= Row("foo", "bar");
rows ~= Row("moo", "car");
Appender!string jsonString;
foreach (row; rows) {
jsonString.put(serializeToJsonString(row));
}
req.writeBody(cast(ubyte[])jsonString, "application/json");