XMLA 1.1. spec for the "Format" property says:
Multidimensional: Indicates that the result set will
use the MDDataSet format (Execute method only).
Native: The client does not request a specific format,
so the provider may return the format appropriate to
the query. (The actual result type is identified by
namespace of the result.)
...but Mondrian supports only Multidimensional and
throws an exception if a client asks for a "Native" format.
Why don't you just return Multidimensional when a
client asks for a "Native" ?
(thus making a Multidimensional format a native format
It conforms to the XMLA spec.
And it is logical for XMLA providers to provide at
least a "Native" format to them, it is the safest
assumption a client program can make.