How to use my dev abilities to harm companies that try to break strikes?

What about a mobile application that can either scan a barcode (not great) or recognize a label (better), and look up a database that contains the ultimate corporate owner(s) of the manufacturer?

From there, you can annotate the corporation records with various attributes: strike-breakers, general bad labor practices, bad environmental practices, political funding data, manufactured by Bangladeshi child labor, uses artisanally-mined cobalt, etc, etc.

The end user can choose which attributes they care about, and get an alert if the product doesn't fit their purchasing preferences?

Maybe not targeting your intents specifically, but what types of tools exist to help workers organize? BLE beacons or covert QR codes to help educate workers on their rights. Geofencing ads to warehouse/factories could be effective, but expensive. Maybe an anonymous app that helps recruit members, track violations, polls over workforce priorities or just explains worker rights and how to educate/engage coworkers. The space is ripe for innovation since there likely isn't much money to be made but lots of good can be done, but it will be hard to get past the anti-union sentiment that most Americans have.

There are a handful of organization apps mostly targeting Walmart that are kind of bland and low utility, maybe even integrating with Walmart HR, so not sure of the objectives there.

The obvious disclaimer to any advice you'll receive on this topic is that almost nothing you can do to harm a large company will be lawful, so you should most likely not even try. That being said, this might give you some ideas: http://theanarchistlibrary.org/library/elizabeth-gurley-flyn...

In general, there's a large body of knowledge in terms of sabotage or monkeywrenching with traditional workers organisation movements, there's no reason they're no applicable to software engineering. Just search for the above words in your favourite search engine.

Build a superior competing product, and be more ethical as you do so.