Overstriking support might be nice, but it wouldn’t help much for this use–case.

If it's not obvious how this could be achieved with overstrike, basically the idea is that you could draw text on a cell then overstrike the cell with 1/8 block characters for the 4 edges characters (U+2581 & friends) to create a border around the cell.

Edit: HN stripped the characters in question from my comment