You mean booting directly to WB without any boot disk?
AmigaDOS requires a boot device, but even modified ram disk (boot capable) would work. Actually some m68k AROS versions did boot to shell without boot disks because RAM disk was accidentally boot enabled
Of course it was quite useless shell.
Some kind of virtual rom filesystem (containing startup, loadwb and some other stuff) probably would work nicely without wasting free RAM.