typedef struct OpalParamGeneral {
...
  OpalMessageAvailableFunction m_messageAvailable; /**< If non-null then this function is called before
                                                        the message is queued for return in the
                                                        GetMessage(). See the
                                                        OpalMessageAvailableFunction for more details. */

...
/** Function called when a message event becomes available.
    This function is called before the message is queued for the GetMessage()
    function.

    A return value of zero indicates that the message is not to be passed on
    to the GetMessage(). A non-zero value will pass the message on.

    Note that this function will be called in the context of different threads
    so the user must take care of any mutex and synchonisation issues. If the
    user subsequently uses the GetMessage() then the message will have been
    serialised so that there are no multi-threading issues.

    A simple use case would be for this function to send a signal or message
    to the applications main thread and then return a non-zero value. The
    main thread would then wake up and get the message using GetMessage.
 */
typedef int (*OpalMessageAvailableFunction)(
  const OpalMessage * message  /**< Message that has become available. */
);

Public Type OpalParamGeneral
...
  'IntPtr
  m_messageAvailable As Long
...
'===
Private Sub CommandSetGeneral_Click()
    Dim NewMes As OpalMessageGeneral
...
    NewMes.m_type = OpalCmdSetGeneralParameters
...
    NewMes.m_param.m_messageAvailable = FARPROC(AddressOf OpalMessageAvailableFunction)
    BackMesPtr = OpalSendMessage_SetGeneralParameters(L, NewMes)
'===
'===ModuleOpalHook===
Option Explicit

Public Function FARPROC(ByVal pfn As Long) As Long
  FARPROC = pfn
End Function

Public Function OpalMessageAvailableFunction(ByVal a As Long) As Long
     Debug.Print a
End Function

OpalMessage * OPAL_EXPORT OpalGetMessage(OpalHandle opal, unsigned timeout);

/** String representation of the OpalGetMessage() which may be used for late
    binding to the library.
 */
#define OPAL_GET_MESSAGE_FUNCTION  "OpalGetMessage"

/** Typedef representation of the pointer to the OpalGetMessage() function which
    may be used for late binding to the library.
 */
typedef OpalMessage * (OPAL_EXPORT *OpalGetMessageFunction)(OpalHandle opal, unsigned timeout);

Private Sub TimerGetMes_Timer()
    Dim MesPtr As Long
    TimerGetMes.Enabled = False
    Do
        MesPtr = OpalGetMessage(L, 0)
        If MesPtr <> 0 Then
            TextBoxAdd TextDebug, "OpalGetMessage: ptr=" & MesPtr
            'здесь предполагается разбор Message зная указатель на него MesPtr

        Else
            Exit Do
        End If
    Loop
    TimerGetMes.Enabled = True
End Sub

Private Sub TimerGetMes_Timer()
    Dim MesPtr As Long
    TimerGetMes.Enabled = False
    Do
        MesPtr = OpalGetMessage(L, 15000)
        If MesPtr <> 0 Then
            TextBoxAdd TextDebug, "OpalGetMessage: ptr=" & MesPtr
            'здесь предполагается разбор Message зная указатель на него MesPtr

        Else
            Exit Do
        End If
    Loop
    TimerGetMes.Enabled = True
End Sub

Public Function OpalMessageAvailableFunction() As Long
     TextBoxAdd Form1.TextDebug, "CallBack"
End Function

TimerGetMes.Enabled=True 'при инициализации, код таймера выше

Public Function OpalMessageAvailableFunction() As Long
    OpalMessageAvailableFunction = 1
End Function

Public Function OpalMessageAvailableFunction(ByVal param1 As Long[, param2 as long,...[paramN as long]]
да хоть бы
Public Function OpalMessageAvailableFunction(ByRef mes as OpalMessage)

Public Type OpalParamGeneral
...
  'IntPtr
  m_messageAvailable As Long
...
'===
Private Sub CommandSetGeneral_Click()
    Dim NewMes As OpalMessageGeneral
...
    NewMes.m_type = OpalCmdSetGeneralParameters
...
    NewMes.m_param.m_messageAvailable = FARPROC(AddressOf OpalMessageAvailableFunction)
    BackMesPtr = OpalSendMessage_SetGeneralParameters(L, NewMes)
'===
'===ModuleOpalHook===
Option Explicit

Public Function FARPROC(ByVal pfn As Long) As Long
  FARPROC = pfn
End Function

Public Function OpalMessageAvailableFunction() As Long 'без параметров-дело темное
    OpalMessageAvailableFunction = 1 'Message помещается в очередь сообщений для чтения
    Form1.TimerGetMes.Enabled = True ' таймер разбудится через Interval=1мс
End Function
'===================
Private Sub TimerGetMes_Timer()
    Dim MesPtr As Long
    TimerGetMes.Enabled = False 'таймер надо усыпить до следующего CallBack
    Do 'чтение всех Message что есть (с перестраховкой)
        MesPtr = OpalGetMessage(L, 0)
        If MesPtr <> 0 Then
             HandleMessages MesPtr 'обработка Message
        Else
            Exit Do
        End If
    Loop
End Sub

Public Sub HandleMessages(ptr As Long)
    Dim MesType As OpalMessageType
    If ptr = 0 Then Exit Sub
    CopyMemory MesType, ByVal ptr, 4
    Select Case MesType 'а вот здесь только решаем нужно ли оно и т.п.
      Case OpalIndRegistration:
        TextBoxAdd Form1.TextDebug, "MesType=OpalIndRegistration"
        Dim s_StatusRegistration As OpalStatusRegistration
        CopyMemory ByVal VarPtr(s_StatusRegistration), ByVal ptr + 4, LenB(s_StatusRegistration)
        With s_StatusRegistration
          Select Case .m_status
            Case OpalRegisterRetrying:
...