Did he say or write something addressing the event? I guess he wasn't very active at this stage, but he suffered a disabling stroke about a year after the riot so I'm interested to know how he dealt with the event.
Chaim Miller, in his biography Turning Judaism Outward (p. 396-397), discussed the Lubavicher Rebbe's response to the riot when Mayor David Dinkins publicly met with the Rebbe, surrounded by members of the press, on the following Sunday (August 25, 1991) after the riots:
At the meeting, the Rebbe appeared calm and was devoid of accusation. Ever the inclusivist, his only point was to articulate a vision of future unity where Jews and blacks would not see themselves as two conflicting communities, but as one single brotherhood. "I am confident," Dinkins assured the Rebbe, "that with the good people of all of our communities, both sides, we will come together and do those things necessary to protect everyone." The Rebbe responded, "We need to forget the 'both sides.' There is one side, one people."
And here is a video of that dialogue between Mayor Dinkins and the Lubavitcher Rebbe.
As to the Lubavitcher Rebbe's contemporaneous reaction in a less diplomatic setting, Jerome Mintz (Hasidic People: A Place in the New World, p. 335) records his address to his Chabad Hasidim:
On Wednesday evening, while rioting continued, the Lubavitcher Rebbe spoke after prayers. Without saying anything directly about what their response to the riot should be the Rebbe referred to the weekly portion of the readings: "When you take the field against your enemies, and the Lord your God delivers them into your power and you take some of them captive..." (Deut. 21:10). The Rebbe pointed out that "When you take to the field" could also be translated as "If you take to the field." His remarks were interpreted to mean that it was inappropriate for Jews to go to war. Rather than commit violence Jews must use restraint and do battle with Torah and mitzvot.