Yeah, this is the problem. While it is certainly possible to get part time employment as a software engineer (I've been doing it for some years now), it's impossible to rise out of the ranks of code monkey. You might make it to senior code monkey but would never become a project leader, for example.
I think this is mainly because the more senior positions need to be across the whole project, available for whenever a decision/problem/whatever comes up and if you're only the 0.5 or 0.6 of the time, you're not going to be very useful.
As noted elsewhere, consulting is perhaps a little different but that has its own pitfalls.