Busybox is a binary that inspects argv[0] to determine how to behave. This is very different from shell builtins that do not launch a new executable, and will

Busybox is a binary that inspects argv[0] to determine how to behave. This is very different from shell builtins that do not launch a new executable, and will thus function correctly after the executable is remove from the file system. As far as I know, you cannot get busybox to perform it's other functions as "builtins". But if you have a functional debugger, it should be possible, though computing offsets is likely to be hard without another machine (chances are that the busybox binary has been stripped).