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
        }
    }
}

If you set Reply Code Actions to :*=R

Retry Interval to 3600

And Failure Timeout to -1

Once a message is sent to the BO it will lead to the message being resent every hour but in my testing there is the downside that you get an error reported for the BO each time since treated as an error even though the message is resent each retry interval and the response is retrieved: