According to this article (https://msdn.microsoft.com/en-us/library/hh230820(v=sql.120).aspx), the MDX query needs to have the following syntax to include restrictions:

Select * from SYSTEMRESTRICTSCHEMA ($System.<schema rowset>, <restriction parameters and values>)

I've tried it with the SPID restriction, but having some issues with the type conversion too.

An interesting line in the documentation for this DMV: "The DISCOVER_COMMAND_OBJECTS schema rowset does not report activity through DMV queries." Not 100% clear on what this means, but it may be the reason for the empty rowset in most cases...