well, Actually I find a bug...
when you create your first instance (object), and for example, set 2 different variables...this will be setting by default (like your suggestion)
after this, when you are making a new instance, the variable would be the same of the old one... but there is a problem, when you remove all of the instance, (appear "you have deleted the last...") and you try to change the variable to make a new default instance variables, (click right in the object on Projects windows), this will not works, contruct2 will load the old variables of the first instance...
so, I think this should be fixed, because you can't actually set a new default instance... If it was possible, you don't need any extra options to make it, but a new options is good, so, I suggest to insert in the properties of the object, just a button set as default instance, (same to the first instance)