A cool tool called Chrome Logger has been around for awhile that allows, through a known specification, client side logging from the server. It seemed to have undergone a complete rewrite recently but only had PHP and Python implementations.
After some quick work last night, I was able to release an early version of a Ruby implementation. It's a Rack middleware packaged as a gem that exposes
env['console'] from which you can create log messages.
It also comes with easy Rails integration:
gem install chrome_logger