We've had couple people hold down substantial client work (more on the order of 20-30 hours a week, I think)
I was typically able to get in 20-24 hours a week while in HS batch[1], so it is possible to do HS and client work at the same time, but it's very challenging. I came in with two clients but wound up passing one of the projects off to another developer because it was too much work. The other client I still work for. Having two clients while in HS was completely impossible. Having one client while in HS is doable, but it depends on the quality of the client. The client that I kept never bothered me about trivial matters, gave me full autonomy over my work, provided the resources I needed to get the job done, and were just plain cool. I still work for them. If my description sounds nothing like any of your clients, I wouldn't recommend doing HS and client work simultaneously.
I was typically able to get in 20-24 hours a week while in HS batch[1], so it is possible to do HS and client work at the same time, but it's very challenging. I came in with two clients but wound up passing one of the projects off to another developer because it was too much work. The other client I still work for. Having two clients while in HS was completely impossible. Having one client while in HS is doable, but it depends on the quality of the client. The client that I kept never bothered me about trivial matters, gave me full autonomy over my work, provided the resources I needed to get the job done, and were just plain cool. I still work for them. If my description sounds nothing like any of your clients, I wouldn't recommend doing HS and client work simultaneously.