Interestingly enough, in designing the escrow, we may want to use some
rather unusual technology, such as sealed boxes of paper, or
destructive read memory, rather than typical computer databases, which
is fun for a database of hundreds of millions of keys.

The greatest danger of the escrow database, if it were kept on disk,
would be the chance that a complete copy could somehow leak out.  You
can design lots of protection, but with enough corruption a complete
copy is always possible.   Storing it on paper or something very hard
to copy at once may actually make sense.  Or a medium that can't be
copied -- for example, something with very slow access times or which
leaves a destructive trail if it is even read.

Of course then it's hard to backup.  However, I think the consequences
of no backup -- the data is not there when a warrant comes -- are worse
than the consequences of a secret backup.

An audit trail that reveals when data has been access, that *can't* be
erased by the humans involved, is also necessary.