I'm building a kernel for an ARM platform running uClinux 2.4 and under "General Setup" in the Linux configuration there is an option called "m68knommu-style attached romfs in RAM support". My ARM assembly skills are somewhat limited but as far as I can tell if I enable this option the ROMFS is copied to the end of the kernels BSS.
What is the purpose of this?
As you rightly indicate, this option causes the romfs attached to the kernel image to be relocated to the end of the .bss section. This allows the system to start from the romfs as its root filesystem.
The above isn't exactly correct; I believe I actually developed the change, if not I definitely used it. As noted, this feature offers support for a romfs filesystem concatenated to the kernel image -- both of which are placed in RAM. Then this option ensures the romfs filesystem will automatically have its size evaluated and be moved to a reserved area of RAM (as well as the appropriate pointers passed for mounting via the MTD RAM driver).
Without this change it is still possible to run out of RAM; you merely needed to have your bootloader place it in a predetermined location and pass in the appropriate kernel options. The big feature this change added was the ability to have a single, unified kernel+filesystem image the way the Coldfire builds did.
Note that it only worked if you have the appropriate changes in your head-platform.S, as I recall -- I think it may only be in place on the NetSilicon NS7520.