Assign action not working on HL7 production ruleset.

Or you could write a custom Business Process to do it. Here's an example with inadequate error processing (😉) that should give you some ideas:

/// Business Process to Modify the MSH:7 field
Class HICG.Sample.SetMSHDate Extends Ens.BusinessProcess [ ClassType = persistent ]
{
/// Downstream processes or operations to send messages to
Property TargetConfigNames As %String(MAXLEN = 1000);

Parameter SETTINGS = "TargetConfigNames:Basic:selector?multiSelect=1&context={Ens.ContextSearch/ProductionItems?targets=1&productionName=@productionId}";

/// Clone, modify and send the message downstream
Method OnRequest(pRequest As Ens.Request, Output pResponse As Ens.Response) As %Status
{
  Set tClone = pRequest.%ConstructClone()
  Set tCurDtTm = ##class(Ens.Rule.FunctionSet).CurrentDateTime("%Y%m%d%H%M%S")
  Do tClone.SetValueAt(tCurDtTm,"MSH:7")
  Do tClone.%Save()
  For i=1:1:$LENGTH(..TargetConfigNames,",") 
  {
    Set tSC = ..SendRequestAsync($PIECE(..TargetConfigNames,",",i),tClone,0)
    Return:$$$ISERR(tSC) tSC
  }
  Return $$$OK
}

/// Return an array of connections for drawing lines on the config diagram
ClassMethod OnGetConnections(Output pArray As %String, pItem As Ens.Config.Item)
{
    Do ##super(.pArray,pItem)
    If pItem.GetModifiedSetting("TargetConfigNames",.tValue) {
        For i=1:1:$LENGTH(tValue,",") { Set tOne=$ZSTRIP($P(tValue,",",i),"<>W")  Continue:""=tOne  Set pArray(tOne)="" }
    }
}
}

The reason you need to clone the inbound message is that Start-of-Session messages are immutable. You must clone them, modify the clone and send it.