The answer is that SSRI's do not work that way, and yes, they have been trialled with the exact predicted outcomes, none. If your serotonin system is working fine, then adding to it will only cause issues.
You could be thinking of beta blockers, which have been used and tested to take during traumatic events, so the event is literally blocked from specific aspects of the brain from becoming traumatic, however; they tested these with little overall effect, because you can't stop the brain processing the trauma at some stage. If you see it, hear it, smell it, taste it or touch it, then your brain records it and must process it. The results only end up in delayed onset...
Yes... they try lots of things, but they are not working. The latest new kid on the block for working is [DLMURL="http://www.ptsdforum.org/c/threads/sgb-ptsd-treatment-article.11417/"]SGB[/DLMURL], though it is an after, not before treatment, and is showing the best results to date of any treatment for PTSD ever.