The alternative, which involves more building, is to make a trailer that can extend.
Imagine, a trailer base frame with wheels that are on pop-put hubs, or QR dropouts. The trailer has two sets of wheel fixing points (dropouts, or holes for the pop-out axles) - one at the half way pivot for the normal short load, and one set at the back end of the trailer. The base folds out backwards.
For small loads, you run with the wheels in the forward position, and the base folded up. For longer or larger loads, unfold the base (you'd need to find a way to support it), move the wheel back to the new centre, and away you go.... (Does that make sense, it's hard to describe without a drawing).
I reviewed a Radical trailer for Velo Vision that had two wheel positions like that, the rear set were intended to allow the trailer to be used easily as a hand trolley, but we reckoned they would work well for carrying longer loads, if the load bed was extended, in effect doubling the length of the trailer.
