Yeah, I've done that.

My most successful projects was many years ago, when water bottle cages came in only one size, I made one that fit a one-liter soda bottle. It was made of aluminum bar stock bent into a kind of helical shape. It was quite pretty, and I used it for years.

A less successful, though more ambitious project, ca. 1982: I mounted a downtube shifter to the stud that holds the extension levers on old Weinmann / DiaCompe brake levers. It actually worked, though never quite well enough. This was before the days of Suntour Command shifters, brifters, and the like.

Also eyelet adapters for an eyelet-free frame (using the triangular hole in the campy dropout); brackets for headlights, not to mention whole headlights, tail lights, etc.