Yeah, have done in the past - although I tend to generate my own, or use a mind machine. I'm quite interested in the whole BWE/neurofeedback scene.
It's rare, and I've personally not seen it for a number of years, but I've stumbled across a couple of situations where there's been 'added content' to certain subliminal/BWE products being shared on torrent. In one case, it was almost certainly the original vendor, and in another case it was just some sicko who had added some rather grim suggestions to a subliminal relaxation soundtrack.
Regarding the whole mp3 versus FLAC situation, it's a fact that mp3 wasn't even designed as an audio compression method, and in my opinion it shouldn't be used to encode audio for BWE or subliminals. It CAN produce audio artifacts that can hamper the designed effect. In some cases, depending on the frequency used for binaurals (for example) it can completely strip out any effect.
:R: