IgG is the basis of post-exposure vaccines.
If you step on a rusty nail, for example, your doctor will give you an injection that will very likely contain tetanus immune globulin.
If you are bitten by a rabid animal, you will be given an injection containing rabies immune globulin.
If you are bitten by a poisonous snake, you will be given an injection containing immune globulin specific to the venom.
This practice has a long history. We were actually doing it before we knew what immune globulins were and those preparations were simply called "serums" back then.
This is not an alternative to transfusion, which is administered for entirely different reasons.
I believe the time lag you speak of does exist with experimental treatments and techniques. PolyHeme, for example was an experimental oxygen carrying blood substitute and it was used on some JW patients in actual clinical trials.
I'm not sure if this is a good example of what you had in mind though. PolyHeme was discontinued around 2009 because of its negative side effects.