Traditional lean startup advice says to keep your full time job while working on your statup on the side, at least until you gain traction...most of all you need to talk to real prospective customers about your product to gauge real market interest, lest you build something nobody wants.
But how are you supposed to do that with a full time gig that has you technically under an inventions assignment agreement?
Those agreements, arguably, would own anything you create while employed, at least if it's somewhat related to your full time job. Granted, their enforceability is questionable in a lot of cases (especially in California)...
But even forgetting legalities for a second, that'd be a hell of an awkward conversation if your boss/team realized that you've been building/pitching your own products on the side without telling anyone. Reactions could range from firing to, most likely, just a general distrust, like you're trying to sneak one foot out the door on the sly.
Maybe some folks are really lucky and could just openly lay it all out on the table, but I don't think that's practical for everyone, especially those in large Bay Area companies.
I know personally if I asked my boss for permission, the 95% most likely result would be for her to put me in touch with legal...and the legal department is by-the-book, so of course they'd never give me any sort of permission or interpret anything remotely in my favor. I may as well just skip that route.
So any advice about the tactics of how to approach this? How to get real potential-customer feedback without risking your full time job?
I try to only sign agreements that match the type of job, i.e. I might agree on a non-compete for 6 months if it is a 3-6 month contract with a paid non-compete period, but not a 6 month non compete on a 2 week job. Outside of a full time job businesses have to pay for their anti-competitive ways if they want you to sign a non-compete/work-for-hire that is not an acceptable timeline. NDAs, work-for-hire when fulfilled with payment are not a problem usually.
Non-competes are destroying innovation and force people to go indie.
In the game industry, contracts there (old school ones) also sign you on to own ALL of your on time and off time products. So you will always see people in the game industry break out on their own when looking to publish otherwise it is owned by the employer or could be.
I would do what you need to do though, no matter what they won't like a worker's time/focus being consumed with another project. Better to keep this one to yourself and do not work with anyone from that job on it, that could get you into trouble. You have one foot out the door now if you are doing this. If you do get big enough for them to notice and it is a problem then you can continue with that path. I wouldn't even worry about them caring about it unless you are directly competing or working with a client of theirs, both of those reasons will be problematic and wrong. And again, do not work with other employees there on this venture. That is where it becomes a very problematic and wrong.