Your mileage will definitely vary on this one. I've tried this a couple of times, and the result was always the same: "Sign an unmodified employer agreement or go find another job."
It doesn't surprise me that mileage can vary; I'm sorry you had to deal with unreasonable companies. Were they tech-focused companies?
In my cases, they were companies where software development was necessary but tangential to what they do. For example I currently work at a printing company where I help to automate digital stuff and help drive print sales with online storefronts. I'm not even sure the owner knows what programming is other than the fact that I do it for him and it makes his stuff work. So he's not interested in owning software I write outside of work hours...
