"Maybe use it for the boot-time 'gather entropy from different sources,' but clearly it should not be used at runtime."

Good idea. Ask it during boot/insmod for some hardware-random bits to seed Linux's usual software-only CSPRNG, then just use that.

And even that might not be a great idea. I wouldn't be surprised if the fTPM RNG is subtly not-entirely-random, at some alphabet agency's behest. I remember there being a controversy over rdrand for this reason…