>> (I also have a habit of assuming that the null pointer as the zero
>> bit pattern [...]
> Also the BSS segment in unix exec images is zeroed to contain all
> binary zero values for the same purpose (i.e. so that the compiler
> doesn't have to include runtime code that explicitly initializes
> uninitialized variables to zero, including pointers).

Yes; an implementation that uses a non-zero bit pattern for nil
pointers will not be able to put them in bss, or else will have to have
a special sort of bss for pointers.  Structs will be messier; they may
have to be moved out of bss entirely.  (This sort of thing is probably
why C is permitted to use the strict def/ref model as well as the
common model.)

> One final note:  even lisp "nil" pointers are generally interally
> represented as all zero bits, at least with the lisp interpreters I'm
> familiar with on unix systems,

I should get you to work on mine. :-)  It represents nil as a symbol
like any other; the nil at the end of a list is, as far as
representation goes, just like any symbol in the cdr of the last cons.

Not that Lisp engine internals are very tech-kern-relevant....

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