What Can I Do With an Aggro Dog?

Why not try tracking?  It's one sport where the dog and handler perform alone.  Tracking is done on lead, so you have added control.  You could muzzle him while waiting your turn;-)  Depending on the course, easier on an arthritic dog, too.  Most clubs hold an introductory session in the spring:-)

 Tracking is a great idea.

I have a friend who had an aggressive Belgian. She got him in to schutzen (spelling?) training and now has complete control of him on leash and off.

Tracking, dock diving come to mind.....

If you like obedience, try Schutzhund or Dog Sport.  These are longer heeling patterns and practical exercises, always outdoors.  There is a second dog that preforms a long down, but the two dogs are not close and are on leash at least for the first levels.  It's not like rally or AKC obedience where the courses are small and there are tons of other dogs in close proximity.

Is it even possible to get a dog who's decidedly not ok with people to a point before he gets much older that I could enter him in some sort of obediance trial for fun? Considering he has a bite history, even with extensive training to make him safe and to a point I *really* objectively believed he was safe and trainer(s) did too would any organisation have us? I

As long as the dog isn't going to disobey you and go running off to attack people you can do most sports because no one, including the judge, is actually going to touch your dog or even get close to your dog. Most sports won't throw you out unless the dog gets written up for aggression at one of their events.

I believe the judges touch the dog in AKC obedience.  The dog needs to stand for exam and the judge strokes the dog.

The nice thing about the Dog Sport and SchH obedience routines is that they are so much larger, longer, and done on open fields.  So even with a judge observing the dog isn't being crowded like in a rally or AKC ring.  My Kenya is not aggressive but can be nervous around certain people.  She does much better and works in much higher level of drive when we are training for her Dog Sport and Schutzhund titles even though rally is way WAY easier (and she does title in rally just fine, but the position, attentiveness, and level of drive are better with the former, I believe because we have so much more room and don't have judges or people on the sidelines breathing down our necks).

 I do AKC rally, with Emma, and we've started obedience and tracking. She may never be able to compete in obedience. It just depends on whether she can get over being on seizure meds, and having a person approach and touch her. She'll never compete in open or utility class, because of the out of sight stays. If a dog were to challenge her, she'd take it on, and nobody wants to see that at a dog show.

I don't know what kind of "aggressive" Rupert is, but Emma is leash reactive, dog aggressive, and extremely defensive. She's bitten people and dogs. As long as things are done with plenty of caution, she does fine. We introduced the stand for exam with a basket muzzle on her. Now, shes fine with ayone from our group doing an exam.