A kill switch to countermeasure border controls?

They're not going to be terribly impressed if they work out you've done this. Consider whether the risk of having this discovered, the risks to you about what you'll need to say when they get suspicious about a wiped laptop, and whether the consequences of getting caught out are worse that the consequences of having CBP gain access to the data you carry across the border...

I wouldn't do this - I'd either wipe/factory-reset the laptop before travelling (and plan on downloading whatever data I need while travelling once through border control), or just take a minimal plausible setup and accept that it'll possibly get searched/imaged at the border.