Export Report pbix via API - after XMLA

We deploy reports through our client workspaces via the REST API and semantic models via Tabular Editor and the XMLA endpoint.

We need to provide the ability for our users who view the embedded reports through our own portal to download the pbix files for the reports.

Currently if we navigate to the report in PBI Service the report can be downloaded with a live connection.

As a result I would expect the REST API to export report also be available. However when calling it against the report I get the response below.
The docs don't mention a limitation of downloading a pbix via API just because it was created by xmla endpoint (ie the semantic model). The only mentioned limitation is 
"For .pbix reports, exporting a report with a Power BI service live connection isn't supported after calling Rebind Report."
Is there something I am missing that I can do to make this download available via PBI?
If its simply not supported do we know if MS are going to make it available?
If the answers to the above are all no - is there any other way I can keep the user in our portal with an embedded report and somehow deliver a pbix file to them?  (I assume there is no way for the embedded report to include a download file option).

Thanks

{
    "error": {
        "code": "ExportData_DisabledForModelModifiedByXmlaEndpoint",
        "pbi.error": {
            "code": "ExportData_DisabledForModelModifiedByXmlaEndpoint",
            "parameters": {},
            "details": [],
            "exceptionCulprit": 1
        }
    }
}