I haven't any ideas for a clamp. But what about putting the 10 LB load
in a wooden box which has a hinged bottom? The opposite side
of the hinge would then be where the pin goes.
You could also try a two door hinged bottom with the pin located in the middle
where the two doors meet.

Well... No word about cost? If cost is no problem, just buy a normal zip line trolley for around 100-120$:
http://adventureropegear.com/cart/index.php?cPath=49_54

If you want go cheaper and dont need rolls you could go for something like a clevis (http://www.oreillyauto.com/site/c/search/Clevis+Pins/N1760/C0142.oap) but that will damage the zipline over (short) time.

Some good ideas can be brought up by looking at existing patents.
As an example:
"zip line trolley" in google patent search gibes stuff like
http://www.google.de/patents/US20110239898
where you see in the figures a detailed plan of every part...
Also the quick-release is visible and may spark some ideas.


I for my part (if i would bild such a thing) would try to make it in one of the following fashions:
- Make a "kicker" where the trolley slides off the side (lift it first with a ramp on the cable) of the cable
- Let the cable "end" in a non-stop: take the cable over a wheel and fix the cable maybe 90° to the ground. If the non-closed (!!) trolley arrives, its forward-momentum will let it ride over the wheel and bent cable thus off the cable.

Both ideas above are only usable if the freight is durable and doesnt break since the stop is nothing short of a crashlanding.

If i would go with a DIY-way, it would look like:
http://picload.org/image/agiproc/untitled.png (i painted that just for you).
The thing is seen from the front and would sport 2 rollers in a line (added stability).
The rolls can be off-the-shelf and the 2 plates (red) can be made/cut from normal steel or even aluminum or whtever you have at hand...
The pin can be of whatever design you find fit.