You should probably look into building a custom kernel with FPU emulation, or at least determine if the reason they say it is not available is that they are speaking of running on top of existing built kernels commonly shipped in routers, or if there is a technical obstacle to even turning on the kernel FPU emulation in the kernel versions that could be used to make a custom build.

You may want to look at LEDE (or even OpenWRT) resources in general, rather than only at Onion-specific ones.