Controlling resolution from lua would be more convenient for sure. I get the impression the hi-res mode was intended to be just a novel undocumented easter egg rather than a seriously supported feature; in one version Zep actually removed it completely from html exports! I was very grateful when he was receptive to adding it back in the next version. :)
Maybe if more people make multi-display carts it might become higher priority? (Writing carts to use it does add another level of complexity though, and can require 2x or 4x CPU compared to a single display cart, so it's not always feasible).