The rules say uniform or conservative coloured/tweed jacket..I think they would not view your burgundy jacket as falling into that category, I could be wrong though
It goes on to say, white, cream or beige jods....gloves to be worn aswell
I have translated the rules as blue or black or tweed / hacking jackets [ obviously depending on the level] and uniforms may be worn where apropriate .
A few years back, people like Anni McDonald-Hall wanted to wear coloured jackets to 'add interest' to dressage but this was 'declined ' by BD. I think at that point it was really decided to keep the conventional colour rules.So i think at affiliated competitions your jacket may not be considered complying with the convention.
But the question is 'un affil' so as far as I know, it's up to the organisers and frankly, at unaffil, they haven't got much scope for complaint. Why not try it and see?
I would think it depends whether the unaff competition is being judged in accordance with BD rules- if it is then I'd say no however if it's not, you may be okay to wear it.