I guess you must be the person I spoke to at Maker Faire about this? Unless skateboard sidecars are suddenly very popular :)
Playing devil's advocate slightly here - I'd just try it. Designing one will only take an hour, and then an evening's worth of printing time and a few pence worth of plastic. Make one, hang some weight from it, see when it breaks. I've had some fairly surprising results load testing some PLA components before. If it turns out to be far too weak, your investment in the idea was minimal anyway, and then plan B can be to mill it out of aluminium/Delrin/something. Load testing it won't tell you much about whether it'll fatigue and snap after 1/2 mile of skating, but it'll give you an idea about whether 3D printing is even nearly suitable for the job.
Danny