Sorry for the very late reply. In case it’s still relevant; yes, kernel 6.15.6 boots from an SDcard perfectly for me but naturally there’s no display output so you’ll likely have to use UART until ssh is setup. Here’s extlinux.conf.gz (421 Bytes) and config.gz (30.4 KB) of a booting system.
The dtb file is generated when the kernel is built and should be copied to the boot partition. For example: cp /usr/src/linux-6.15.6-gentoo/arch/riscv/boot/dts/starfive/jh7110-starfive-visionfive-2-v1.3b.dtb /boot/dtbs
As mentioned in Experimental Gentoo Image - #127 by rvcray you might want to edit the dtsi if you have a 8GB model. While an 8GB model will run without editing the dtsi doing so means it’s only able to use 4GB.