Professor Friedman has posted various amicus pleadings on his blog here. He lists one not currently on the SCOTUSBlog for the case. A little more here with Supreme Court cases with potential impact on military justice. Professor Friedman does note as to his own filing:
There are a couple of passages that I would amend if I could, in which I refer to the "primary purpose" test and appear to assume that it governs a case like this. I believe a "reasonable anticipation" test is far preferable, and I am hoping that the "primary purpose" test will eventually recede, perhaps first by being confined to interrogations. The recent decision of the Michigan Supreme Court in People v. Fackelman,, which I expect to discuss in a post in the very near future, raises my hopes that this may turn out to be true.