The David S. Wyman Institute has posted an exhibit about the Rabbis' March on Washington.
It seems that the Rashag was the Freidiker Rebbe's delegate and that a very young Nissin Mindel accompanied the Rashag.
Update: The S.Z. Gourary on the list is not Shmaryahu Gourary (a.k.a. The Rashag), the son-in-law of the Freidiker Rebbe. It is the very young Schneur Zalman (better known as "Jimmy) Gourary.
The Freidiker Rebbe (probably due to illness), the Rashag and Rebbe did not attend. It appears that no senior Lubavitch rabbis attended . . .
The March, attended by all major leaders of US Orthodox Judaism, including many long-bearded Hasidic rebbes and Lithuanian sages, is credited by historians with the creation of the US Government's War Refugee Board. The Board saved 250,000 Jews during the last 18 months of the War.
Wealthy, establishment non-Orthodox Jews opposed the March:
According to an internal Bergson group document, some [pro-Roosevelt] Jewish Congressmen "had done all they could to dissuade the Rabbis from making their bearded appearances in Washington." Representative Sol Bloom of New York, who chaired the House International Affairs Committee and was a staunch supporter of the State Department's refugee policies [i.e., leaving Jews to die in Nazi-controlled Europe], led this effort . . . "At a certain moment they almost succeeded [in dissuading the rabbis] but Mr. Bloom spoiled the soup by telling one of the Rabbis, as an additional inducement for not going, that it would be very undignified for a group of such un-American looking people to appear in Washington.
Nissin Mindel was clean-shaven at that time.