Finding a job where you don't sign off your intellectual right on secondary works has been my limiting factor.
You may request an exception, but it'll need to be reviewed and approved beforehand by their lawyers.
Previous discussion here: https://news.ycombinator.com/item?id=2208056
Now, in some states (WA but not CA or NY) your employer can fire you for moonlighting if they want to but that's about it. You also have to watch out if you signed a non-compete and are arguably competing with your employer.
California CA Labor 2870 -2872
Delaware 19 Del.C. s 805
Illinois 765 I.L.C.S. 1060/2
Kansas K.S.A. 44-130
Minnesota M.S.A. 181.78
North Carolina G.S. s 66-57.1
Washington RCWA 49.44.140, 150
Utah UT ST s 34-39-3
[1]: https://www.ieeeusa.org/members/IPandtheengineer.pdfLittle bit more on that: https://news.ycombinator.com/item?id=9807612