there isnt any official boot loaders.
the closest I have gotten to no Onion OS's are Debian Mipsel (theres a guide on setting it up in a chroot)
There is another guide on Installing FreeBSD on the omega , using Uboot and an LMZA image.
But other than that. its the stock Busyboxish openwrt . (which is good for getting Iot devices working)