Self-experimentation with within-subject design can be internally valid (I make sure mine are well-powered, even, which is more than some psychologists can say), but this does nothing about external validity or selection bias.
Which ironically makes self-experimentation somewhat analogous to quantum suicide experiments: because of selection bias, observers of my self-experiments will rationally learn little even as I learn much more. Someone watching quantum suicides will expect to see lots of survivals, and someone watching self-experiments will expect to see lots of positive results, even if quantum suicide doesn't work and self-experiments measure nothing but null effects.
(Except maybe if the observer had some reason to believe they would have learned about my experiments regardless of the experiment results... possibly because they became interested in my writings for non-experiment reasons, maybe? I wonder.)
When you say "can be internally valid" what do you mean? What about interactions from repeated treatments? I mean, correlation can equal causation, too. But that's a pretty weak standard to meet.
Also, how do you know the selection bias does not create non-causal explanations for observed dependence? For example, in case control studies you select based on the child of the outcome:
T -> Y -> S, with unobserved U1 being a parent of T, and U2 being a parent of Y (U1, U2 possibly dependent creating unobserved confounding).
If we select on S...
http://www.johndcook.com/blog/2013/02/04/four-hours-of-concentration/
And since this is the Internet, and facts are involved, our gwern turns up there also.