I'm still a bit stunned that the longlat assumption goes so deep in leaflet and elsewhere. Discussions about "auto-projection" get hijacked by paranoia that users will get caught out by mistakenly assigned input metadata , as if "garbage-in, garbage-out" is somehow a surprise (we don't shy away allowing folks the responsibility to get their epoch in order before setting time zones, for example). It's unfortunate perhaps that Mercator is more or less the same topologically, but they are the only sane choice for a global tile set, and is ultimately why we support those weird cases, with otherwise sensible choices that are somehow "exotic".
This will change fast as WebGL and kicks in and we see attention on data structures suited for 3D engines, so I'd caution against spending much effort on workarounds. Consider that tiled imagery was absolutely state of the art in the early 2000s, with a huge amount of experimental and short-lived projects that then got swamped when Google bought Keyhole and then sub-licensed the entire world's aerial and satellite imagery. We saw a revolution of that first tiled web imagery era when leaflet came out, and now we see WebGL and "3D tiles" and D3 (arc node topology with its dynamic curvature) bustling around the edges of the "geo-spatial" world. It's a good time to be sure that the new 3D and 4D masters have a good understanding of map projections, as well as all the other coordinate systems and "projection" (in the mathematical sense) that are used in science and engineering.
R/sp and grid/ggplot2 support generic CRS very well but only when you're "on your own" with a metadata-free graphics device. Ironic eh.
I only wish we could see prospects for the window/s we put our work in to fully support it, even if they only support manual over-ride and "pass through without interpretation". "mapview" is currently the only sophisticated window into which we can throw semantically-rich objects, and have it just work - but I gather this isn't an assumed basic feature for most GIS-y users anyway.