We need to persist the metadata object as well. So one option is to have 

bd.webapp.OutboundMessageRequestMetadata Extends (%SerialObject, %JSON.Adaptor)

then with the following %ShowContents (note Write changed to Do to remove status output of 1)
 

Method %ShowContents(pZenOutput As%Boolean = 0)
{
    If$ISOBJECT(..MessageContent) {
        Set json = ..MessageContent.%ToJSON()
        set formatter = ##class(%JSON.Formatter).%New()
        If pZenOutput {
            Do formatter.Format(json)
        } Else {
            Write json
        }
    } 
    If$ISOBJECT(..Metadata) {
        do..Metadata.%JSONExportToString(.jsonExport)
        set formatter = ##class(%JSON.Formatter).%New()
        Write:$ISOBJECT(..MessageContent) ",",!
		If pZenOutput {
            Do formatter.Format(jsonExport)
        } Else {
            Write jsonExport
        }
    }
}