- Refuse to take action. They want engineering done, they can bloody well do it themselves. Don't type a single keystroke in the direction of helping them.
- Announce what is going on anonymously. Plenty of avenues for this.
- Announce what is going on, publicly. See if they do indeed want to take you to court.
- Quit.
- Take down the service. Much easier if the service is only a part of your company. Helps a lot if you don't retain the information they're looking for.
- Lie.
- Destroy records (this is by far the riskiest action here, above a simple public announcement).
- Delay. And delay more.
- Keep information outside the jurisdiction, possibly controlled by a third party who will not comply with orders.
- Misunderstand ("Is that a one, or an ell?")
Most of these will get you into trouble, a few won't. Most of these are really difficult roads.
I've given this some thought before. If I found a backdoor in a product, I would remove it with a tracking bug and a checkin, and send internal email about a really bad bug that I'd just fixed; the more internal people that know, the better. And if a VP showed up and berated me, I'd just tell them to fuck off, and quit if it came to that.