I'll wade in to what seems dangerous and sticky territory, and I'm sure Boban will find reason to scuttle my input!
I had a feeling it was OK to use someone's design, or elements of, if you acknowledged the source, like referencing a quote. To say you've made a "Rietveld zigzag chair" is to unequivocally admit the design is someone elses, and you're not pretending to take credit for such. Rocker's trouble with permission from Maloof probably sinks that theory!
I know with quoting/copying text from books and journals for academic purposes, there is a limit of something like 10% per chapter, with suitable referencing. So maybe a similar proportion of the design can be used, again with acknowledgment:D!
Maybe the problem lies in the intent of the reproduction...to make money, to enhance a career, versus being made for your own use, or simply to research technique.
Cheers!