Orange Pi 5 systems will get a kernel and U-Boot update with next DietPi update, which should also enable NVMe support, after flashing the new bootloader binary to SPI via dietpi-config > Advanced Settings > Update SPI bootloader: Orange Pi 5/ROCK 5B | Kernel update and migration by MichaIng · Pull Request #6532 · MichaIng/DietPi · GitHub
USB boot works fine afterwards, would be great if some could test it with NVMe. We should then add an entry in our documentation about this and a link from the download page.
New images with this kernel to test are available here: Index of /downloads/images/testing
Another bonus is that this kernel should fully support the Orange Pi 5B as well, i.e. its onboard WiFi.