I stored an Excel.Application instance in a listview item .Tag property without error but it may be just the String of the objects name.
Code:
Option Explicit

Private Sub Form_Load()
    Dim itmX As ListItem
    Dim oApp As Object
    Set oApp = CreateObject("Excel.Application")
    ListView1.ColumnHeaders.Add , , "Test"
    Set itmX = ListView1.ListItems.Add(, , "Test")
    itmX.Tag = oApp
End Sub