Repack the filesystem image from vmlinux.bin (embedded initramfs) without rebuilding?

Yes, it is possible but changing the .init.ramfs section size and addresses is not enough because the Kernel's ELF executable is statically linked with the virtual address of the start and end of the initramfs' section.

In the Linux sources the relevant code is located in the iniramfs.c source file:

void __init populate_rootfs(void)
{
  char *err = unpack_to_rootfs(__initramfs_start, __initramfs_end - __initramfs_start, 0); 
...
}

So you also need to change these two offsets in the machine code of the invocation of the unpack_to_rootfs() function, which is located in the .init.text section. ( watch out for any relevant entries in the relocation table! ...if one exists )

Also, in reference to Icarus's reply, the manipulation of the initramfs' section' size, file offset and starting virtual address, as well as these two aforementioned offsets (arguments to the unpack_to_rootfs() function), enables you to add you own custom LARGER initramfs section that is loaded ABOVE the maximum virtual address of the ELF file. The Program Header's (PHeader) "Memory Size" field needs to be modified too, in order to reflect the larger initramfs section appended after the end of the old virtual address space.

P.S. The "hole" in the Kernel's virtual address space remaining after moving the original init.ramfs section to a new high starting virtual address, does not hurt anything because the associated memory is later freed by the free_initmem(void) function defined in the init.c source file.