At present the only external freeware I'm using in my Oxted line is the superb Lanky track (the flat bottom and bullhead 03 version). I know these are hosted here so it's relatively easy to put a file association with them.If the assets are freeware there remains the enormous problem of locating them since there is a wide selection of sites which may host them.
We have discussed this before and it seems unlike MSTS where everything had to be included in the route folder, there's a bit of reluctance to bundling third party items in a Railworks route pack even when permission is freely given. I'm not sure why this is though I suspect there's trepidation over accidentally zipping up some payware items, also a couple of us recently noticed issues with the packager where it wasn't gathering the required texture files for an object automatically. This even applied when taking an item from one of my own previous routes which was in a separate folder. If you've created the model yourself, then you probably have an idea what textures need to be included with it but one made by someone else no clue at all. Ergo, it's easier to just say, "Look here and download it yourself".
Going forward there was talk a while back of community scenery "packs" which would at least put several items in one place but this seemed to die a death?