There is a "/proc/self/exe" support under GNU/kFreeBSD, but it is limited, namely inside combination of bind mounts and chroots.What's the problem with /proc/self/exe anyway? It works fine here, even inside chroots. Did you nullfs-mount /proc?
No, I expect that the problem is when the executable is inside bind mounted dir and there is some chroot setting. The exact setup of buildd is unknown to me. Petr