Hi Rob
Unfortunately, AFAIK, that is the standard behavior of the IDoc. In the past, we have implemented a custom process code, which uses a custom function module to process this IDoc.
That custom function module could either be a modified copy of IDOC_INPUT_MBGMCR or a wrapper around IDOC_INPUT_MBGMCR. In this function module, you can then tweak the output message in the IDoc status return table.
If all you want is just to find the associated document, you can branch to the document by using the relationship link functionality in the IDoc monitor screen.
Rgds
Eng Swee