I guess the government could argue RICO at that point against the lawyer, but I think that still wouldn't abridge lawyer-client confidentiality. Maybe it would.

Then the lawyer couldn't delete the systems like Lavabit did because the government would have confiscated them as evidence of the conspiracy.

At that point, however, the 5th Amendment should apply, because the server provides evidence of a conspiracy the lawyer took part in, providing the encryption keys would also be providing evidence of the conspiracy; requiring the lawyer to testify against himself.

Some judges have tortured the 5th Amendment worse than this, so it's not entirely foolproof. Still, it's unlikely other lawyers would take this sort of abuse standing down, nor would the general public.

So even the most extreme legal logic I can think of would be unable to penetrate this arrangement, providing the computer security and encryption was all top-notch. Disclaimer: IANAL, though I'd be willing to be the IT employee for any legal firm that wanted to construct such a system.