The only justification mentioned so far for changing Braille from So to Lo is to be able to use Braille in identifiers. I'm not sure why someone whould want to use Braille in this way, for a start how would these identifiers be translated into Braille?
Braille identifiers only make sense when the whole source file has been translated to Braille. However, the parsing semantics applied to it should then be determined by the properties of the original characters (before applying the Braille mapping). If one does want to work directly with a Braill transcoded stream, then such systm must support *dynamic* property assignments. That's something that's outside the scope of the Unicode Standard.
In conclusion, it seems that the correct set of *default* properties for Braille would be determined by the needs of inserting Braille strings into other text (for educational manuals and similar specifications).
As Marco has pointed out that means BIDI = L and I believe it also means GC=So, and other properties assigned as they are for other characters that share BIDI=L and GC=So.
A./
