No statute defines "clean room RE" and there's basically no relevant litigation establishing case law.
This "clean room RE" fabrication is also not something so obviously compelling that nobody would ever attempt to challenge it - it's just that nobody has cared to put dollars behind it. IP provenance cases are enormously difficult to prove and are almost always settled ahead of trial. This concept is software engineer folklore more than anything.
I seem to recall DeCSS being the 'hot button' as it was RE'd in a 'non clean' way, the code released, and then a 'clean room' RE could be made from the 'public' info ??
edit: actually, at the time I think there was German law allowing RE for 'interoperablity' or some such. But that was a long time ago so my memory might be off.
It's safest to err on the side of plausible deniability, any proximity to OEM software increases liability if things go wrong.