Yes, 640×400 could be added on the mode list. The parameters on this page look correct (mainly: 449 lines, 800 dots per line) so I’ll use those. Separating it from 720×400 would be hard as both are 449p, but it should quite easy to adapt from 640×400 parameters (h_total -> 900, h_active -> 720) e.g. using a different profile when necessary. I’d assume most people prefer using DOS PC with a monitor, so there’s better chance of 70Hz compatibility than when using a TV.