Bear in mind the draft horse was DESIGNED to do one thing... pull (well, push, really, as a horse pushes into the collar, not pulls from it - but that is semantics)... and do it slowly, all day.
Everything from the angles of their bone structure to the massive weight they carry is aimed at strength, in a slow, even gait.
With this in mind, jumping a draft (and I am talking a purebred, or cross which takes after the draft parent) is only asking to injure it, in the long run. Whenever you ask a horse to do something is was not designed to do, you increase the chance of injury to it.
The hip/pelvic angle of the draft is not ideal for jumping. It is ideal for working slowly, with a lot of power.
The angles involved with the hindquarter construction of the draft are also designed for working slowly, with a lot of power... not for the ability collect, thrust and support the whole weight of the horse.
The bone the draft has is large in comparison to a light horse, but is proportionate to the amount of body mass they have - so they do not have the "big bone" which is desired for jumping/dressage/cross country... those who DO have large bone for their breed type, are likely too heavy to be good jumpers anyhow.
The shoulder and neck of a draft horse is designed for the job they were bred to do as well, not for jumping. The length of neck, the thickness of neck and the slope of the shoulder allow a draft to jump, but it certainly won't make an "ideal" jumper.
Jumping puts added strain on joints, it stresses the back, and one wrong approach (even over something so "tiny" for a big horse) can lame the horse for life. Why take that chance on a horse which isn't bred for it in the first place? Yes, drafts CAN jump... yes, many of the "lighter" crosses even do well in jumping classes (the heavier ones, you'll often find, later on, with arthritis, bone chips, stifle injuries, foot problems etc.)
I'm not trying to be a downer, but this has got to be one of my biggest equine pet peeves. In all honesty, if you want to jump, get a light horse with the conformation to complement that job - otherwise, love your draft for what he/she was bred and is easily able to do.
I own drafts (and I do ride one of ours - the other is my husband's and he drives/logs)... I love draft crosses for riding - provided they have the conformation to support that job... I am in no way "anti draft"... but I do think it's important to understand the biomechanics of the horse, and how it applies to the job you're doing in order to protect the long-term soundness of your horse.
And before anyone says "I only do it once in awhile"... bear in mind it only takes ONCE to do serious damage.... especially if you don't or haven't taken jumping lessons under a qualified coach.
I know of a 18hh Clydesdale mare who is now permanently lame because her owners wanted to jump her (did something to her stifle jumping a 2' oxer), I know two Percherons - one PB the other a HEAVY cross (He's a QHXPerch... but very stocky) one with severe arthritis (was shown up to 2'3) the other with a few soundness issues (was jumped twice... he was lamed while being "tried out" for a new home... something happened to his knee, and hock)... a PB Belgian who used to do little baby stuff at home, once in awhile, stumbled after a jump and bowed a tendon which never healed quite right. To me... totally not worth the risk.