There could be some cross-reactivity of antigens with antibodies, but this occurs with decreased effect. Antibodies are like complex locks, looking for the key that fits them. They are very specific to a very specific key. Kind of similar keys might kind of work though. However its likely that our own antibody production would do so better by random chance than any HIV specific antibody would. Since we are not all being cured by this all the time (though I might pose some caveats to that line of thinking) its highly unlikely it will help.
Having said that there appears to be a rare subgroup of HIV patients who have something ME-like. We know this because of one un-named patient who has a bad showing on the two day CPET. So there might be people with undiagnosed HIV who might benefit. However, and this is my experience, when I mention my symptoms to a new doc, or at a hospital, one of the first things they get me tested for is HIV (and most recently also for blood alcohol, which is kind of a joke). I have been tested for HIV so very many times. What are the odds all of those tests are false negatives? How is it that, even with probably decreased lifespans, we are not rapidly dying with HIV? Its very very unlikely any such drug would help.
Here is what might bring it home. When a virus mutates even a little, a therapeutic antibody might fail. Indeed this is how viruses, like HIV, evade the immune system, at least in part. You develop an antibody, one or several nucleotides mutate in the virus, and then the body no longer has a useful antibody. Yet the human immune system has a huge variation of antibodies available to it. Any synthetic antibody is very limited into what it binds to.
What we might be able to do, theoretical so far I think, is clone our immune cells in vitro, introduce the pathogens, then note the immune response. Yet to duplicate the complex interacting factors found in immune cells inside the body might be a very difficult task.
