I don't think leaving a piece of carbon fiber post in the seat tube is a good idea. Graphite is at the very top of the Galvanic Series and can corrode even stainless steel if in contact and in the presence of moisture (of which there is plenty in the Seattle area). This appears as a problem in the sailing world where carbon fiber spars will corrode away the stainless steel terminations on the shrouds and stays that hold the mast upright - with dangerous and expensive consequences!