Hi Joachim.
Thanks for the reply, that was indeed the problem. I must confess that I posted the question before looking into the problem very much. It was getting late and I wanted to get the question posted in the hope that you or Marten (given you are five hours ahead of me) would answer with some ideas before I got back to working on it today.
The root of the problem stems from the fact that the packaged likes to remove methods it doesn't think are needed. When using the Seaside #on:of: method to connect an html input element to getter/setter methods, only the getter method is specified. In this case #rememberMe. When Seaside needs to set the value it assumes the setter is named the same as the getter with a colon ":" as the last character. This goes unnoticed by the packager, so it removes the method. If I had used #rememberMe: elsewhere it would have been included.
I added a few packaging rules of #doNotReduceSubApplicationNamed: to reduce the packaging my Seaside apps. This was the easiest thing to do and had very little effect on the size of the packaged image, so I'm happy. Thanks again.
Lou