Wouldn't it be better to use blocks (219 in ASCII) as the main paint unit and use some other half-block and "hashed" characters (ASCII 220, 223, 176-178) to implement dithering?
Everything above 127 would be extended ASCII instead of strict ASCII, if that matters.
There are multiple extensions to ASCII. "Extended ASCII" isn't a single thing, but a category of single-byte character encodings which are supersets of ASCII.