I hope someone can chip in more on the details.
The stretch test not only acesses the strenght of the strap, but more importantly the plasticity of the material, as it is worn over time,temperature and humidity, the strap may deform and alter. Homologation done in house allows close charting of these elastic parameters which will feedback into IWC product improvement as the watch ages and evolves over 2008 thru 2010.
Many brands do these tests "out house", but the disadvantages are that information would no longer be exclusive, much information done inside IWC homologation department has allowed them a certain wisdom, exclusive to the brand. Much of this can be seen in the fact that they were able to make the Cal 5000 series, a huge full sized automatic rotor, something that today's modern watches do need, but has not been done en masse.