I don't know of what science or practical research it may be based but like you have read much from manufacturer's recommending baby should be at least one year or able to hold their head up on their own before riding in a trailer.
Personally, we put a car baby seat into our trailer, strapped it in and put our baby in it to go out from when she was a few months old. I figured (and I may be wrong!) the ride was similar to being in a pram or push chair, especially if you're careful about riding on good surfaces (which you are when you are pulling such a precious thing behind you) and keep the tyres soft to give a little suspension. We did lots of day rides and then used the same configuration for a 350 mile two week tour one cold Easter when baby was just 6 months old. And it was great.
Sure, some people see it as reckless and will tell you not to do it, people certainly did with us, but I do think there is a lot of ultra conservative thinking about this and manufacturer's needing to cover themselves too. Like any aspect of parenting, you need to assess the risks as best you can, and consider the benefits to you and the child, reduce and control what risks you can and then get out there and show your kids the world. It's certainly better than putting them in a car seat and driving around.
Stuart
www.familyonabike.org