While with enough force you could pirate a sea-stead, bear in mind that it would be a lot harder than the piracy you hear about in the news. Part of the reason dirt-poor Somalians can have success pirating is that technology has made it possible to staff a massive sea-going structure with the outline of a skeleton crew, possibly under a dozen people, which can be overrun by six armed guys on a boat. Piracy against a sea-stead would be more like mounting an armed assault on a remote village at the very least, with an existing police force, one probably created with awareness of the possibility of piracy. You can't just grab a gun and clean out your local downtown without encountering some... issues.
And if the floating sea-stead is a libertarian paradise, everybody's probably armed in which case your cost/benefit odds are pretty bad....
The real threat is less random piracy than sovereign invasion. The good news is that the list of countries with a credible ability to mount a military assault on a seasteading platform is short. The bad news is that it is also basically the exact list of countries that the seasteading effort is trying to get away from. To me, that's the biggest problem with the idea, it is intrinsically based on the sufferance of the very entities you are putatively trying to avoid, and that's basically true regardless of the politics of the platform.