I now believe I misunderstood the IRS statements about what an "interest" in a foreign asset is.
The first paragraph under the headline "Interests in Specified Foreign Financial Assets" on page 6 of the IRS Instructions for Form 8938 essentially states "you have an interest... if any income [etc]... would be required to be reported... on your income tax return". The second paragraph adds "... even if no income [etc.] ... [is] included or reflected on your income tax return...".
I originally thought the two paragraphs taken together meant that whether you were required to report information about it on your tax return or not, because you had signing authority, you had to file an 8938. Now I believe these two paragraphs mean you only have to file an 8938 if you had sufficient interest, i.e. that you could potentially receive income or suffer a loss, that you would be required to report on your tax return if you actually received income or suffered a loss, whether you reported an income or loss on your return or not.
