I have not seen the book, but I have had a lot of experience baking deserts in Quetico.  I'm sure this method will work, but it will be time consuming and the results will be marginal.  The problem is that the walls of the outside pot are just too thin and either do not hold the heat or get too hot.  Your cake will be burnt on one side and not done in the middle.  This is the same problem as the disposable aluminum baking pans (see post to Cranberry).  The GSI aluminum ovens solve all the problems.  The pebbles on the bottom is the true answer.

I have also used a bedowin (sp?) oven and this also works.  It is very light weight and funcional.  I bought mine from Indiana Camp supplies but don't know if they are still in business of if it is availabe.