A much better alternative to this would be to for more terminal emulators to implement overstrike mode (DECSET 20, as available on the VK100). That way you could just draw border characters over existing text cells to achieve the same thing.

Unfortunately mintty is the only terminal emulator I'm aware of which supports overstrike [0], as this would open up a world of new possibilities for TUI apps if more widely supported.

[0] https://github.com/mintty/mintty/wiki/CtrlSeqs#overstrike