I did and all I found of relevance was an article almost two years old when the bowling flexion limits were 5 degrees and even then it says;
After the tests, conducted with state-of-the-art technology at the Biomechanics Laboratory in UWA, the conclusion in the Bowling Report is that "Mr. Muralitharan be permitted to continue bowling his `doosra' at least until a valid data base is collected on the various spin bowling disciplines. The relatively minor level of elbow extension following remediation over the period from arm horizontal to release is not believed to give Mr. Muralitharan an unfair advantage over batsmen or other bowlers".
That's a direct quote from the UWA report.
http://www.hindu.com/2004/04/29/stories/2004042905562200.htm
The response was that obviously 5 degrees for spinners and 10 for pace bowlers was too small a margin considering measurements of mean flexions of most bowlers so the decision was made to increase it to 15 degrees. That was common-sense. At that point there wasn't a data set which was extensive enough to determine what the flexion limits should have been. Once the data was collected, the 15 degree limit came along.
Even the original UWA report was done before the 15 degree limit came along;
http://www.rediff.com/cricket/2004/may/15murali.htm
Either way, still no mention of anyone saying it's impossible to bowl the doosra inside of the limits. Again, an example?