The thinkpad support is mostly done by the free software part of the coreboot community, and they don't touch devices that come with a Management Engine (see http://me.bios.io/).
If "security conscious" includes not having any unexplained binaries around, the Asus C201 Chromebook may actually be the best bet (maybe there will be devices with the same chipset but more laptop-like properties eg regarding storage), but it's less powerful than a contemporary Intel based device.
If "security conscious" includes not having any unexplained binaries around, the Asus C201 Chromebook may actually be the best bet (maybe there will be devices with the same chipset but more laptop-like properties eg regarding storage), but it's less powerful than a contemporary Intel based device.