The memory allocated by xmlDumpMemory must be deallocated (free()'d).  The Zend/zend_API.h macro RETURN_STRINGL() doesn't do this.  A substitute (below) seems to do the trick.

#define FREE_RETURN_STRINGL(s,l,duplicate) {        \
        char *__s=(s); int __l=l;           \
        return_value->value.str.len = __l;  \
        return_value->value.str.val = (duplicate?estrndup(__s,__l):__s);    \
        return_value->type = IS_STRING;     \
        free(s);                            \
        return;                             \
    }

I used this at the bottom of domxml_dumpmem() and can now do bunches of dom functions without crashing Apache or the CGI.