Hi, Mik
The short answer is not directly as you said but you can benefit the fast NVMe speed.
Because RK3399 can not boot directly from NVMe. It can boot from SPI/eMMC/SD directly. There must be a bootloader in SPI/eMMC/SD to load boot images from NVMe. Currently the rk3399 u-boot doesn’t have NVMe driver, so we have to wait the kernel boot to use NVMe. You can put the rootfs in NVMe and bootloader and kernel in SPI/eMMC/SD.
So to use the fast NVMe, you can put the whole rootfs in NVMe and have bootloader and kernel in microSD card or eMMC module.