Huh? What's the difference between a methodology and an approach? Oh, wait. Maybe you answered that in your next sentence. An "approach" is an unwarranted assumption, made without any methodological demonstration of its validity. Is that about right?
Now, leaving aside your mischaracterization of the scientific method as inclusive of an "assumption of chance" (or, for the purpose of narrowing the issue, accepting that mischaracterization as true), what would your "assumption of design" accomplish?
Since you dismiss out of hand any need to demonstrate by analysis that an object under scrutiny is designed (and simply declare it designed by fiat), what purpose does your declaration of design serve in your subsequent investigation of the object's lineage or constitution?
Frankly, the only purpose of such a declaration that I can discern is the commencement of an inquiry into the identity of the designer. But of course that inquiry is, according to ID proponents, either off limits entirely or, at a minimum, of no interest.
I wouldn't characterize either of those as necessarily true. As I understand it, ID proponents merely acknowledge that there's as of yet no scientific data to go on that would give any solid indication as to the identity of the designer. That doesn't mean that it's of no interest scientifically, or that it's "off limits", and it also doesn't necessarily mean that no progress will ever be made in that direction.