I've done a little bit of locums during fellowship as a moonlighting opportunity and so I've learned a bit about how the process works, the pluses and minuses, etc. I'm certainly not an expert but I feel I've got some insight into your question.
I think it is certainly possible to make a living doing what you describe, and it obviously is because there are many people doing exactly that. As smurfette said, there are different types of locums jobs, from more consistent part time jobs to more random grab a shift here and there kind of things. Most of these jobs are available through one of the dozen or so Locums companies out there, which are basically like temp firms, they collect a bunch of doctors in various specialties, then hospitals come to them when they need coverage and they coordinate. Working through these firms was nice for me since it meant I didnt have to do any work for licensing, credentialing, scheduling, pay, anything. It was basically like "Can you work this date? Ok, show up, and we will write you a check." These are usually paid hourly, or some version of "$X for the first Y hours, then $Z/hr for every hour after that," but there are lots of variations. As with anything like this, MAKE SURE YOU READ the contracts and the fine print and know exactly what you are getting into. It is important for you to know what kind and how much malpractice they are providing (or if you have to provide your own), things like that.
The downside of firms like this is that obviously they take a pretty huge cut, and they operate in kind of a sleazy way. Its like working for a used car salesman. They dont care about the patients, they dont care about the hospitals, they dont care about the doctors. They care about filling as many shifts as they can. And any given hospital may give the bid for their open shifts to like 5 different locums firms, so they are very competitive and aggressive with each other. Its a little unseemly.
I think it is possible for you to do this independently, and if you were seriously thinking about doing it for a living, permanently, I would strongly recommend looking into that option. Its definitely a lot more work but I think the pay would be markedly better as well.
In terms of how much you can make and how lifestyle friendly, a lot of it is going to depend on how flexible you are and what you want. If you are willing to fly anywhere in the country on relatively short notice and cover whatever shifts people have, you can actually make quite a bit of money doing this. Its hard on you if you have a family or crave stability but as I've done it over the last year or so I can definitely see how a certain type of person/personality would be able to enjoy it.