According to to the US Supreme Court in the US Airways Inc. v Barnett (2002), the answer is generally no. In the court's view, the seniority system will prevail. When the requested accommodation conflicts with seniority system, that accommodation is not "reasonable", unless the employer shows that making such an exception to the seniority system would be appropriate.
Obviously, the above language from the highest court leaves a lot of grey area for arguing which exceptions to seniority systems are acceptable and which are not, and every situation has to be evaluated on a case by case basis, in light of its own unique facts and circumstances. Sure, the needs of both, the disabled employee and the other employee who is supposed to be reassigned or promoted based on seniority will have to be balanced in making that determination.