They don't need negotiating power. Qualcomm can tell Google to shove it -- the Android code is still published under the Apache license and as long as they don't claim to have written it themselves, they're free to do whatever they want with it.

Google could certainly tell LG/Samsung/HTC that a condition of getting the next Nexus deal is that all binaries must be able to be freely distributed by Google and let them put pressure on the SoC vendors, but I doubt the Nexus line sells enough or drives enough profit for anyone to take such a threat seriously.