Ah ok, cool.
The current rematerializer design actually holds up to two shipments -- one visible in the output buffer inventory and one "still cached somewhere in the energizer matrix" :) which streams into the output buffer as space becomes available.
The alternative rematerializer design would be to wait until the output buffer is completely empty before starting a cycle, but since the dematerializer can start refilling its input buffer while a cycle is in progress, the current approach seems balanced.