Rabbi Shimon Eider's book, Student Edition of Halachos of Tefillin, dates to 1985. In it (section Ⅲ D 7), he writes:
Some Poskim hold that one should sit while putting on the Shel Yad and stand while putting on the Shel Rosh; this is the minhag of Sefardim. Other Poskim hold that one should stand while putting on both the Shel Yad and Shel Rosh; this is the minhag of Ashkenazim.
The claim that Sephardic Jews customarily sit, and Ashkenazic Jews customarily stand, while donning the hand-t'filin accords also with what I have always understood to be the case (for what it's worth). There's a little more information on this practice elsewhere on Mi Yodeya.
However, in the past year or so, I have seen three or four Sephardic Jews in Ashkenazic synagogues donning the hand-t'filin while standing. While it's possible that they have decided to follow the custom of the community in which they're participating (an idea with halachic basis in some contexts), I wonder whether there is more to it than that. Specifically:
Has any Sephardic rabbi ruled, specifically citing the custom of sitting to don hand-t'filin, that Sephardic Jews should ignore that practice and stand (either generally, or in an Ashkenazic synagogue)? What are the details of this ruling and who issued it?