I'm not entirely following this thread (I haven't chewed on the code much) and I don't use DevX controls, but a couple of thoughts.
First if all you want to do is set some defaults, I believe I'd just try setting them in the constructor of the sub classed control. This might not work, but I think it will. If it didn't I'd likely try overriding the create control method. Any properties you set on designer would then override them.
Second, if that didn't work or wasn't what you wanted, then I'd use shadowing to replace certain properties with my own. This can get tricky when dealing with inheritance and interfaces, but is definitely doable.
Here's some sample code for the first method (just typed in):
Public Class dxTxtC
Inherits MicroFour.StrataFrame.UI.Windows.Forms.DevEx.TextEdit
Public Sub New()
'-- Set any defaults
Me.BackColor = Color.Black
Me.Mask = "9999.99"
End Sub
End Class
That should be the entire class! Of course I might be missing what the issue is or some other issue your having but this sort of thing is pretty easy.