How handy are you with a sewing machine? Maybe make decorative "sleeves" for the reins, to give them that "tournament horse" look? You could do something similar if you have a breastcollar, sort of make a cover for it that could be put on and removed without damaging existing tack.
I've always wanted to try this, but never had enough material: If you can get your hands on enough wide strap leather (like, old leather tie straps from western saddles), you can make a decorative breeching, like the piece over this horse's rump. http://www.suitability.com/v/vspfile...%20costume.jpg
I personally liked the idea of using small rivets (putting felt on the horse-side for protection, of course) to put the thing together, and leaving it natural leather for a more "functional" look, but the fabric looks good, too.
Please post pics as you make progress, and of course of the finished product!!