Ah, but there's the rub...
ISPs who are "discreet" in how they wish their infrastructure to
be "viewed" will continue to engineer methods in which portions
are not visible to the public at-large.
Somehow, I don't think that will ever go away, so trying to tilt
at windmils w.r.t. (paraphrased) "...what interface an ICMP foo
is sourced from..." is, indeed... tiling at windmills, methinks. 
Cheers!
- ferg