I see your point, and I realize that the rules protect this method of action, but I still see it as exploiting an oversight moreso than anything else.
I have my purify elixer in my potion bag, and some thick cranberry juice which is being used to physrep blood. Flamingo Scavenger-Vampire Lord Mortegue walks into town, sits in the tavern, and demands the blood of Schkvelein the earth mage be served to him in a glass or woe betide the town who would defy him. The highbies are busy with the magic sword getting mod, and the only ones defending the tavern are me, Schkvelein, and afew other low levels, making an offensive reaction out of the question (he has our number in flamingo-ghouls). Thinking quickly, I grab Schkvelein, drag him into another room, tell him to scream while I empty my physrepped blood into a glass, physrep emptying my container of purify potion into a glass, and then empty my physrepped blood into the vial to represent the contents. I then walk back out into the main room, and offer this glass to Mortegue, the liquid looking, tasting, and smelling just like blood IG, because theres nothing established that purify potions can't taste and smell like blood, and its not alchemical, so Mortegue's scenting ability is useless to detect it. Hell, I dont even need the cranberry juice. I can use water and say it tastes/smells like blood, because thats my intended look and smell of purify elixers, and no one wants me drawing my own blood to physrep it. Potions dont need physrepped liquid to begin with, so I could even hand him an empty mug and tell him oog that there appears to be blood in there. Their are no rules governing this type of potion/elixer use, and theres a lot of room for exploitability in just allowing them to be as big, look and taste as you want it to.