From the pc demos faq( http://www.oldskool.org/demos/pc/pcdemos.faq ),
the ET4000 is considered a defacto standard: "(Many older demos use special hardware tricks that might only work on an ET4000; the ET4000 is considered the demo "standard".)"
As for games, the one I've been looking at is dawn patrol. It has support for two variants of et4k along with nine others vga chipsets but no s3. There's a fallback generic vesa driver but it wants exclusion of video address space from emm386(If there's a way to do this in dosbox I missed the blurb on how to do so; having searched vogons for an answer.)
Though I think getting the emm to handle inclusion/exclusion or some alternative should definitely be on a todo list, if not yet supported, since there's enough programs that make use of/require this like in dawn patrol's case, allowing the use of the generic vesa driver with the current s3 emulation, I'm still leaning towards ET4000 emulation for it's direct support.