apart from it perhaps being too complicated
I suspect this has quite a lot to do with it. Another factor my be reluctance to "dilute" the LCC concept of selling you a cheap seat, no frills, no changes or flexibility, use-it-or-lose-it.
But if I were running easyJet, once I had finished with the purges and executions of everyone involved in the speedy boarding debacle, I would introduce a system of ticket buy-backs. Every ticket would have a buy-back value, i.e. the price which easyJet would pay to buy your ticket back from you. This would change over time, and customers could track it online. Most of the time, the buy-back value would be zero, or some nominal sum to make it worth the customer's while to click the mouse a couple of times so the seat could be returned to inventory and re-sold. But in cases where a flight was subject to unexpectedly high demand (as in Anansis' example), the buy-back value could be raised accordingly - up to, or even beyond, the original price paid. In this way, easyJet could in some cases roll over an entire passenger load, buying back all the original tickets at, say, the original purchase price +10%, and re-selling them at a 100% mark-up or more.
This idea is essentially a way of allowing the yield management system to respond to actual as well as anticipated demand, even at very short notice.