Old spokes wrapped in some electrical tape should keep the hitch linkage stiff but less than entirely rigid.
I LOVE the pneumatic quick release as a mechanism for an axle level trailer hitch…and so does my wife. The dumb-ass vice-hitch that came with the kidarooz trailer was repeatedly failing. So I took inspiration from my BikeRev.com EcoShopper and crafted my own hitch. A pneumatic couple cost me about nine bucks and the 6″ of 3/8″ hose with a pressed fitting was prolly another $7.
On the bike I zip tied a 5″ piece of copper pipe with the male quick release attachment. If the zippies show wear I will upgrade to hose clamp.