As the father of a child with a disability, it is really hard for me to say this... but she needs to work to improve this situation rather than be let on the team. Why don't they invent some sort of contraption that doesn't require a dog to be on the floor?
If it is 4 lbs, i am sure they could rig a camel back or something to carry the tank. Judging from the picture, the thing looks very small, almost like a hairspray can or something. I am sure they can work something out.
I fear they are babying this girl due to her disability. It is something that is VERY hard to avoid, but it is critical that you challenge your child rather than baby them. She is 17, she can figure this out (I am assuming her disabilities are physical, not mental).
I say make the girl create a device that she can wear that will not put others in danger. If she can design it and create it, let her play. Shit, let her play with a real pick-up team, not the special Olympics team (which is mostly comprised of children with mental disabilities, not physical, right?).