I still don't completely understand why you can declare a Read-Only Property to a Constant but I found a way to achieve my goal with a Property Get.@Fluff - But it's Read Only. How is it not a Constant?
Option Explicit
Sub TestCont()
Dim fn As String
fn = "Book1.xlsx"
Debug.Print ThisWorkbook.Path; ps; fn
' Or
MsgBox ThisWorkbook.Path & ps & fn
End Sub
Public Property Get ps() As String
ps = Application.PathSeparator
End Property