Just a couple of more thoughts.
There is a set of controls that allows a block of text to be ignored by a screen reader. Might be perfect for allowing ASCII Art on the Grid. I think ASCII Art in general, for some people, might be preferable for some people who want to preserve immersion in a text based medium. Although perhaps I'm wrong on that.
That's the relevant link, although there are several other states of the aria tag that would also allow alt descriptions to be included as well.
Another thought that I had, which was kind of goofy was that allow images... of ASCII art. The biggest complaint about images seems to be that some people don't want to see an image that contrasts to what the look text, or their imagination has built up. ASCII art kind of obfuscates details with a low res grain. Perhaps allow images, but they need to be made of ASCII art... why? Because the IMG tag in a browser already supports the ALT tag for screen readers.
Just a couple of thoughts.