Is there a clear equivalent of OpenWRT-based PirateBox (LibraryBox) in BSD-land?
OpenWRT is a Linux distro, and the Linux kernel is huge. The benefit of OpenWRT is that it runs on nearly everything (almost everything!), including gizmos that fit easily into your packet. Which is a crucial requirement for the PirateBox (LibraryBox).
On the other hand, while the BSD kernels are much smaller than the Linux kernel, BSD projects aiming embedded systems (ARM, MIPS) only support and a handful of devices, none of them fit into your pocket as easily as the popular TP-Link travel router and it's derivatives (an OpenWRT favorite). Most BSD router projects aim x86/amd64 systems, which do not fit into your pocket so easily. Yet.
Alternatively, if no embedded (ARM, MIPS) equivalent is found, how would you make an x86/amd64-based BSD PirateBox (LibraryBox)? We just have to wait a few years while x86/amd64 systems will match the size of the popular TP-Link travel router family. Alternatively, you may try an older, Intel Atom-based Android phone, ASUS made those, then good luck with the drivers.