Property Get/Let で定義したプロパティって
ByRef渡しで値変更はできないのな

'Class1.cls
Dim n As Integer
Public Property Get Value() As Integer
Value = n
End Property
Public Property Let Value(newValue As Integer)
n = newValue
End Property

'Form1.frm
Sub Hoge(ByRef n As Integer)
Debug.Print n
n = n + 1
Debug.Print n
End Sub
Private Sub Command1_Click()
Dim c As New Class1
Debug.Print "Before "; c.Value
Call Hoge(c.Value)
Debug.Print "After "; c.Value
End Sub