"The pointer version still has to test for the nul character anyway, right?"

Some architectures set the zero flag on memory loads. I think the PDP is one of them. Google gives me https://groups.google.com/forum/m/#!topic/net.lang.c/LKAYbzX..., which claims that

    while (*ra++ = *rb++);
    

compiles to

    L16:
        movb        (r10)+,(r11)+
        jeql        L17
        jbr         L16
    L17:

If you do i != len, you need an extra comparison instruction.