It's a good idea and consistent with the original use of the term on HN. I guess the downside is if anyone with a cool side project thought that it wouldn't belong under that smaller-sounding umbrella.

Another option would be to go the whole hog and explicitly reference side projects in the title, or at least in whatever will become the instructions at the top of the regular thread.

That's good, but hacking for me has a specific connotation in that you are modifying something for your purposes. Something new is "working".