No modifications need to be made to the small object allocator. The malloc situation was just an example, it does not mean that setting pointer bounds occurs only in a malloc call. It also occurs when you declare a static array or when you initialize a new pointer:

int *x = (int*)malloc(10 * sizeof(int));
int *y = x; // the bounds for x will be passed on to y

When using "fcheck-pointer-bounds -mmpx" _all_ memory accesses will be instrumented.