No, I'm not talking about that. I'm talking about David Scott's bijective BWT variant. It sorts symbols by a slightly different type of context, and so can do away with a sentinel symbol or a stored index entirely. It was described in the wikipedia page last I checked.

Scott was a bit of a weirdo, but his algorithm actually got recognition in academia, there have been many papers written about it. One character saved doesn't make a lot of difference for compression, but the transform being a permutation does give it some nice properties.