100% leakage, and not via valves or into the cooling system?
Maybe if the rings are broken and with pieces missing. I'd be suspecting a hole in the piston.
I'm with Schurkey -- 150 psi compression test *and* 100% leakage into the crankcase
don't jive/hard to explain? On the other hand, if the 7 other cylinders give you the
expected results (verifying your test gear for you) ...then this last cylinder could be
visually checked for a hole in the piston with a cheap (free to rent) un-articulating
borescope.
And if you happen to discover a holed piston, taking a pic of the borescope display
and posting it here will net you much praise from the remote troubleshooters reading along. :0)
But good on you for trying to gather as much data on your ailing big block as possible.
As a fellow 454 owner, I'm rooting for a successful outcome.