Maybe the limitation is between Powershell and this CIM provider implementation (And not related to ESXi, actually it will be interesting if you can install the same provider on a windows machine)
PowerShell 3.0 shipping with Windows server 2012 and Windows 8 brings a new set of Cmdlets to manage any server or device that complies with CIM and WS-Man standards defined by DMTF.
Seems to be that
http://www.dmtf.org/standards/wsman
-ResourceUri<Uri>
Specifies the resource uniform resource identifier (URI) of the resource class or instance. The URI is used to identify a specific type of resource, such as disks or processes, on a computer.
A URI consists of a prefix and a path to a resource. For example:
http://schemas.microsoft.com/wbem/wsman/1/wmi/root/cimv2/Win32_LogicalDiskhttp://intel.com/wbem/wscim/1/amt-schema/1/AMT_GeneralSettings
By default, if you do not specify this parameter, the DMTF standard resource URI http://schemas.dmtf.org/wbem/wscim/1/cim-schema/2/ is used and the class name is appended to it.
ResourceURI can only be used with CIM sessions created using the WSMan protocol, or when specifying the ComputerName parameter, which creates a CIM session using WSMan.
We have a namespace, "micron/cimv2", but we never had to declare a resource uri. This isn't required for use with WBEM.
So maybe you have managed to access this namespace with a "WBEM" protocol, but to have this namespace available via GET-CimInstance it must be designed for WS-MAN protocol according to WS-MAN standards.