AFAIK, GNAT and GCJ (GCC front ends for Ada And Java respectively, the latter now dead) implement null pointer exceptions by simply raising a language level exception from the SIGSEGV handler (after I assume checking it happened in user code instead of the runtime).

Completely non portable of course, but with the help of glibc, the Itanium ABI exception support and non-call-exceptions it can evidently be made to work reliably, at least on Linux.