Even a mixte or a trapeze frame still has to be leaned over to get your foot over the crossbar. In situations like this the weight of the bike (and what you're carrying on it) also comes into play, so it is not just about the stepover height, which is influenced about an inch and change up or down by the tyre size. A bike that looks like a mixte with an extra tube, called a crossframe, can be made stiffer than a standard diamond frame; I have a cross frame rated at 170kg of luggage in addition to the rider (no weight limit) and the bike itself.
The ideal thing, for a really low stepover, would be an ouma (granny) or oupa bike (the Dutch may spell these as oma and opa), called a "wave" frame by the clever marketers; this is like a big U with without any crossbar, for stiffness without gross weight probably best made in big section, thick wall aluminium. Here's an extreme example:
http://backpackingwithjesus.com/tag/danish-bikes/ Check the Dutch makers like Gazelle for what's easily available commercially. You don't often see one of these set up for touring, but if it the frame is stiff enough, I don't see why not, as in The Netherlands many of these carry two or more kids on the back plus the shopping and get passed down through the generations.
I doubt you'll be offered a wave frame in carbon; be tough to make plastic stiff enough. My fave would a be wave framemade from an alloy block machined down to a cross-section H frame on its side, with the head tube and seatpost insert designed and machined in. Or it could be made lighter in industrial foamed ali in a mold, a process proven by Biomega and Marc Newson, see:
http://marc-newson.com/mn-bicycles/ Notice how the head tube, bottom bracket shell, and rear frame end (mono) are all part of the single construction, no welding, no sticking-out bits. A low-stepover version would look something like a tilted-over W with a single central upright as the "seat tube". Having said all of that, and confessing to be a techno-freak, I looked into the MN back when Biomega first sold it, and found it so almost comically impractical (where would you attach mudguards, lamps, etc?) that eventually I just walked away.
