There are other opportunities out there for part-time flying gigs, but like all flying jobs they are not always easy to find. I know a couple of people that have full time jobs and do some flying on the side.
One guy works a factory day job that ends around 3:30. He meets the Part 135 experience requirements (1200 hours total among other) and works as a fill-in part timer for a local freight hauler.
Another guy I know is a full-time fireman that flys part-time 135 charter for the outfit I work for. I think he was flying full-time as a youngster, before switching to firefighting as a career. That's how he got his foot in the door.
I worked for more than a year as a part-timer myself. I had built up enough hours flying myself for business (I was part-owner of a Twin Comanche for a few years; i.e. lots of multi-engine time) that I talked the charter company into using me as a part-time pilot. I would take vacation days from my day job to fly charter trips.
There are two difficulties to getting part-time gigs like this. The first is building enough time to become employable. The second is developing the contacts and being in the right place when the opportunities become available. I think that doing part-time CFI work is a great way to get both of those.
Keep in mind that in order to do part-time work for most employers, whether cargo, charter, corporate or others, requires a great deal of flexibility in your schedule to meet their needs. Too many phone calls where you answer "I'd like to but I have other commitments today" and you'll find yourself off the call list. Remember that there is usually a cost to the employer involved with your being on-call - things like training, check rides, paperwork, drug test costs, etc., and they need a reasonable return on those costs otherwise you are not worth the expense.
