ROSv6 is based on linux kernel which pre-dates NVMe hardware, so I don't think ROSv6 will ever work off NVMe. ROSv7 might work, though it's in late alpha / early beta stage.

v7 will have much better support for most of hardware, including x64, support for more than 2GB RAM (also on CCR2004 IIRC), many more supported peripherials under i386/x64 (storage devices, 10Gbps+ NICs, broadband USB sticks, ...). Could well be that lack of x64 support in v6 is due to proprietary drivers and MT didn't want to make them 64bit-safe. With v7 it seems MT will go with stock drivers (or chipset / peripherial vendors' drivers).

CHR is completely different beast because there's VM hypervisor providing abstraction layer taking care of real hardware and presenting virtual hardware to VM. I guess NVMe support is depending on chipset support as well ... quite many lower-end SoCs don't support NVMe, hence no support on those models in ROS.