Hi Ian,
I, too, applaud you for your dedication to your donkeys. I have trained 2 mini donks to drive (I used to be minidonkeyny here), I am no expert, but have had a bit of experience w/ longears.
They are different than horses- that's for sure. I agree check the breeching...you want a handwidth between her and the leather- like your pinky on her- your forefinger on the leather...not just slipping her hand flat.
Check the bridle...I had one specially made for my guy- and it went with him when I sold him. I got it from the Iberian connection..but several places could help you there...browband and enough space behind the ears is important...
They will never fill out like a horse...harness fit was always tough w/ the donks...one of mine would be fat on air- the other always was more angular....
The bit mine worked best in was a mullen mouth liverpool...they have a shallow palete so watch a typical snaffle....
I would be curious to hear more about the special donkey bit you were talking about. I took a driving clinic in VT (Asspirin Acres) years ago which wsa a ton of fun....I dont know if they still offer it or not.
Keep in mind to that donkeys are big listeners...those ears back some of the time could be concentration and listening..would be easier to determine in video..
Here's what I finally decided with mine. One was never really comfortable driving single-and the other was really into realxed driving...not schooling in circles and working on transitions....so I ended up selling my boys to a wonderful lady who raised Paints...She just wanted something fun to play with with her grandkids- and calm and quiet was fine by her. Last I heard she was getting them working as a team- as the one felt more confident working w/ his buddy...They get very bonded to their friends...SO I sold them together and the lady loves them. It was a better situation for us both, as it wouldn't have been fair to put him into a mold in which he wasnt happy.
And I bought my BuckWheat...he was 9 months at the time and is now 3 and we are enjoying the journey of his training together.
Good luck and have fun....hope I helped a little.
Angie