There has recently been significant discussion in the United States on the use of foreign law, and foreign legal proceedings, being used as a means for informing a domestic judge's rationale for deciding a case. Justice Breyer for example has come out in favor of the idea saying that judges can be informed by looking at the logical process and decision making efforts of judges in other countries with similar laws when deciding their own cases. Legal opinions at all levels already routinely cite foreign law as background.
However, given that every case has a unique set of circumstances and foreign law is not legally binding on any United States citizen, could a lower court's opinion be invalidated on the grounds that foreign law was too heavily relied upon (assuming it was not directly cited as precedent)? Is there any discussion of this practice in the code of judicial ethics? Finally, is this a potentially unconstitutional practice in its own right given that the Justice's constitutional duties limit them to "interpreting the constitution"?
NOTE: This question is distinct from an analysis of domestic laws and potential conflicts with treaty obligations (like this question) as treaties originate from within the domestic legal system. This question is only meant to look at wholly foreign laws, governing foreign peoples.