At one hunt I saw a Shetland with a very creative running plait. I suppose your main concern is the section D's long, thick mane?
If you really would like to plait, perhaps you could try a running plait, instead of the classic dressage one. Everybody will understand that you had made a real effort.
I also saw a very brave lady with what looked like a Welsh cob, and she had plaited him up, mane and tail. The roses were "a little" on the wide side, but they held fast.
For safety, if it has a long thick mane I’d plait it (or jog it). I once got my hands caught up in a horse that had that much mane, it was on both sides of his neck. We were going flat out and I was terrified of what would happen if I fell off
I've always plaited for lawn meets, even the full hair traditionals usually come out with a running plait. Mine has a thick and wirey mane so if not pulled that short just had lots of plaits. Why does one activity have to 'win'? Just adapt to what you are doing on the day?
Perhaps check with the Hunt? I only go out with Bloodhounds but they do seem to prefer everything to be plaited, even if it's a running plait on a long / thick mane. I do however say this as the owner of a Welsh D who is phobic about plaiting / his mane being touched in general and so I ask for permission to not plait and just make sure he's clean and well groomed. Someone apparently plaited him too tight in his youth (previous owner said they took hours to remove... wasn't her that did them but a friend I think) and he's never forgotten... Any attempt to even THINK about plaiting is met with hysteria. I literally can't even pick up a strand of mane without him starting to freak out... flipping annoying thing is that he naturally has quite a thin mane that tends to stay fairly short (yeah any form of pulling / trimming / even cutting with scissors is also not tolerated) and it would plait up so lovely if he'd let me.