The BAPI_DOCUMENT_CECKOUTVIEW2 BAPI requires some programs delivered with sap gui to be run on the client (in this case the nw/mii server) machine to receive the file. Also I think when calling the bapi with the .net or jco connectors you need to explicitly set some application parameters to allow the connector to run these programs, which I don't think are exposed to us by MII.
The SDOK_PHIO_LOAD_CONTENT Bapi basically does what the ABAP snippet Sam Posted does, which is pass the document back as a base 64 encoded string.
I have been around a bit in the MII space and I have never seen or heard of a vanilla MII solution based on BAPI_DOCUMENT_CECKOUTVIEW2 working. It would be really cool if the RFC listener idea worked, I don't have time to try it out so I hope you do and let us know if it works out.
Regards,
Christian