Dependent Text box parameter defaults

Hi.
Each report I make is about a given instance of the overall problem. So we pass in the number of the instance as a hidden parameter. For various dynamic selection parameters, I use the instance number to populate the list of choices without any problem.
In addition however, I have some text box parameters where the user needs to be able to pick any number (s)he wants. There are a lot of these, and it's not always clear to the user what value is a sensible default. I have a Java method which returns an array of defaults based on the instance number, but can I use it to set the default values of the other parameters (given that it has a parameter as an argument)? Since the first one is hidden, it seems possible BIRT should know to evaluate it first, but I guess this might be delicate.

Any suggestions greatly appreciated.

Thanks.