The last mention of any service I can find in Messerschmitt Me 321/323 The Luftwaffe's "Giants" in World War II by Hans-Peter Dabrowski is Dec 1944 on the Eastern Front in IV./TG 4. More were destroyed in 1945 on airfields but do not appear to have been used for anything. The heaviest single unit I can find it ever having carried was a Marder SPG at 11 tons. The most advanced version that had any actual design work on was Z Me 323 F and that could only carry a max of 18.2 metric tons.
If you like the plane Dabrowski's book is the best on it that I've found yet.