I have been unable to follow the trail of these captives, however, my guess is that they were paroled on site and dismissed.

He entered Macon, Ga., on April 20 [1865], but has not yet found it necessary to destroy any properry there. At this place he captured Generals Howell Cobb, Gustavus W. Smith, Robertson, Mercer, and Mackall, and over 1,000 prisoners.
OR V47, Pt. 3, p. 357
.......

Major-General Wilson has captured Macon, with Major-Generals Howell Cobb, Smith, and Brigadier-Generals Mackall, Mercer and Garrison. p. 836