It can be used, and works extremely well. The problem, as you understand, is that in order to work it requires root access, which grapheneos does not offer and does not support.
Having said that, if you have a pixel 6 or lower and keep your bootloader unlocked, you can "fastboot boot" a modified boot.img to achieve root temporarily/periodically to perform backups.
I'm not going to address the security implications of anything related to bootloader unlocking or root, that's all your business and your risks to take.