This issue of accented letters not used by the foreign language is touched upon in the current Interim Manual. This is much more clearly and specifically stated in the new foreign language rules now under review by BANA. Here is the direct quotation:
1.3 Types of foreign language texts
a. Entirely foreign text. Transcribe such texts using uncontracted braille and the appropriate foreign alphabet symbols as provided in Rules 13 and 14.
NOTE: If an accented letter occurs which is not part of that language, use dot 4 before the letter. Insert a transcriber’s note immediately to explain the format.
Unless the text identifies that second language as Hawaiian, it is not appropriate for the transcriber to assume that it is, although I happen to agree with you and I think that it is too. However, we do know that you are working in Spanish and the letter in that name has a non-Spanish accented letter. Use a dot 4 accent indicator and simply state in the TN that the accent indicator is used for accented letters that do not occur in Spanish. If this occurs more than once, consider a transcriber's note on the TN page, rather than an individual TN every time this occurs.