Can't say I ever heard of anyone trying to do this before, but I can make a suggestion. Send it to the shipyard sponsored by your trade world and put it in orbit. Then launch it. Will a menu appear that asks you to assign it to the world you want it assigned to? Anyway, this is what I would try. I have no idea whether it will work. If you try it, please let us know if it works.

If I remember GC2 correctly, in that game you could put a freighter in orbit of the planet you wanted to start the route from then launch it. Need to try to remember to try that also if the orbit & leave connected shipyard doesn't work.

This is very handy to know.... i pretty much gave up building freighters at all since they are so incredibly slow to build and so incredibly slow to arrive anywhere! just a massive slowfest all round really (early game when you actually need them).

However, if i can build them at a manufacturing world and move them to the economic world, well, that changes everything!

Oh, but what about if you upgrade a random ship to a freighter, will it remember the planet that ship was built at or will it default to your homeworld?

I tried that in a recent game.  I sent 2 freighters to my trade world.  Put them in orbit then ejected them and sent them off to another Civ's planet.  However in the trade screen the point of origin for these routes were still the shipyard colony not the the trade colony.  

There should an option to click for changing the point of origin. 

IIRC the freighter will change it home planet only if you eject it from planet that sponsoring shipyard. If shipyard is within 6 ranges of any planet and you eject it out, it will let you choose one from all planet within range of 6.