Rough rule of thumb for max lengthening of an aluminum hull?

That is very evasive. There is nothing counter -intuitive about cost/benefit analysis, but the opposite. It is a tabulated comparison of several alternatives. We do that for large projects. Sometimes they are after a disaster and the response has to be as fast as possible. For example, three winters ago the whole northern area of Nebraska had ice damage throughout. The ice sheet was up to 4 FT thick and moved like a glacier. Almost all the bridges were down and most of the roads. Bulding were also destroyed. We did the initial damage inspection, focusing on underwater, streambed and stream boundaries. That report was the foundation for the cost/benefit analysis on repairing the infrastructure and future modifications to prevent the extent of the damage. Any engineer is familiar with that kind of analysis.