I have also been thinking about how this may be achieved. I downloaded the e-commerce sample database from WebAssist, which is very helpful, and have built a complete back-end for the store using PHP, including adding product option groups and options.
The problem I have is that there seems to be no easy way of getting this to work with eCart. I know there was reference to future tutorials using this ecommerce database and think a tutorial on adding product options in eCart using the sample database would be an excellent idea. I love the design flexibility eCart gives me, but have yet to use it on a live site due to the inability to add dynamic product options.
Pretty please :-)