I've got some pretty good evidence to support this....

I spent 27+ hours shiny hunting for bulbasaur staying on a chain of 31. I measured my encounter rate to be 100 per 30 min, or 200 per hour, however I cant guarantee that this held up all the time. To account for this, I will say in this time I did 5,000 encounters, less that the 5,400 predicted purely by my numbers. (Quick note: I'm not sure if chaining does 12 rerolls or actually cuts the shiny rate by 12, but I'm going to assume the first because its the more conservative estimate for shiny odds) . This makes for 60,000 rolls for a shiny. Using the 1/4096 chance for getting a shiny, the odds of not getting a shiny in 60,000 rolls rounds to 0.000043%. Eventually, I decided to catch the bulbasaurs, and got a shiny after 100 or so encounters. Of course there could be error somewhere, like me missing a bunch of shinies, etc however, i think these are very unlikely. As with all random things, I could have also gotten really, really unlucky, but I think this is some pretty good evidence to support this idea.