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The smallest QR codes are 21x21 pixels. Braille is 2x4. So you might be able to do it assuming there are the right patterns.

What about data matrix [1] instead of QR? The smallest symbol possible is 10x10 so that should be tweetable.

Braille characters occupy all 256 (for the 2x4, 8-bit matrix) combinations, so invalid (unused) characters do exist.