It's not that hard to harness train a bunny (in my opinion)
If the rabbit doesn't "want" to take the harness on, (bites the neck part of it etc) give it something to eat while you attack the harness. With most bunnies I've tried this on, they hardly notice the harness is there once it's on.
Don't try to lead them like a dog, it won't work. In the begining its best to let the rabbit just go as it pleases wearing the harness, perhaps just let it go in the run without a lead, so it can get use to it.
All my bunnies have been harness trained since they were 3 months old (Zakura actually wore one for guinea pigs when she was 5 weeks:roll: but that was before I found out she should have been 3 months) but I've tried it on adult bunnies as well at school. Tried once on a one-year old rabbit and once I put her on the grass, she immidately started bouncing around. This was her first time ever wearing a harness.
NKHF has regulations on how the harness is supposed to sit. Some use so-called "chest-harnesses" which puts pressure to the chest, rather than the neck. Others use "H-harnesses" Named so because if you open them up they look like H. Also the ring to fasten the lead on has to be on the back, not in the middle of the harness or on the neck-ring.
Here's Aroma in her "Bunny Sports" harness:
Also does are not allowed to compete if they are pregnant or nursing. The rules say they can return 10 weeks after giving birht, 2 weeks after the babies usually stops nursing. I think it's 2 weeks if the babies die.
I wouldn't put a harness on a pregnant doe anyway, not sure about nursing does but to be on the safe side I'd rather not.