"Maybe there's a stable region or set of regions you can target (in which case the high overall mutation rate doesn't matter). Or a cocktail of sequences, specific to the global HIV population (I doubt this, HIV mutates more in a single individual than Flu does in the global population)."

Hmm... I would bet that there is a cocktail of sequences such that if they are not conserved, the virus effectively becomes no longer HIV (no longer infectious, no longer capable of producing symptoms...).

HIV is obviously not a human being, right? Find every sequence where it differs, target them all. :-)