First time I hear of the linux-libre kernel and thus I never tried to use it.
After reading a bit about it I don't see any real problems in using that kernel, but I would be careful and try the following scenario first:
Build a normal, stable LFS (version 6.6 is the latest stable at the moment). With normal I mean: Use the kernel that is recommended in the book. Once you have it build and it works, compile the linux-libre kernel and point an entry to it in the grub menu ("dual boot" original and libre kernel). This way you can fall back to your original kernel if needed and tinker with the linux-libre kernel.
If that works, then you can try building an LFS system based on the linux-libre kernel and headers (see this LFS chapter
Hope this helps.