I don't think my friends would have a problem giving me a truthful answer if I said, "hey, I'm doing an experiment this week; I need you to tell me if I smell or not".

Right...

Just like the last time you encountered body odor or anything else annoying in public and immediately, openly confronted the person about it, exactly as you would have if they had approached and questioned you with a "doing an experiment disclaimer".

No, most people I know would give an honest answer, since they'd be the ones to suffer from it and it's easily fixable (as opposed to being fat).

hahaha, I think you underestimate the level of crassness, frankness, and rudeness my friends and I have with each other.

I don't think so.

It's pretty typical for crass, frank, rudeness applied to on subject to be completely acceptable while speaking freely on another could easily end a friendship.