Bob, I think you had the general concept correct, I just want to add some caution to anyone thinking of taking that approach. An immune system will develop a 'memory' if it is constantly exposed to the pathogen (it's not the only way, but the one you were referring to), so if someone is drinking from a lake or stream and not getting sick, they may not be exposed to any pathogens and not developing an immune memory.
This is *if* an immune memory can for for giardia. It's seems like it can (and also for cryptosporidium), though I don't think it's conclusive yet. They haven't developed a vaccine (but wouldn't that be a true test of a ULer!) & part of the problem is giardia has a pretty big (~190) variety of surface proteins it can express. Your memory immune cells are thought to 'hold' onto degraded pieces of these proteins for fast recognition and response of the adaptive immune system. If the protein is different, it might not be efficient (depending on how similar the coat protein structures are).
It also depends on how much of the pathogen you consume, which you don't really have control over. Too much could overwhelm. One thing anyone could do to help reduce the risk of infection (not just from giardia) is to make sure they have ample, healthy commensal gut bacteria, ie. acidophilus, etc. as they will out compete the pathogens for nutrients and essentially starve them.