I've long wanted to try to repair a cracked frame by sleeving the crack in carbon fiber. 30 mm overlap, 1-2 mm laminate thickness and you should be good to go if you've done the prepwork right and gotten good adhesion.
Two things comes to mind:
1) cracks like that tend to come from fatigue. How much life is left in the rest of the frame?
2) you'd need to lose the DT shifter bracket to be able to do the best possible sleeve.