To sum up this thread . . doesn't seem to be such a thing on the market. Best solutions:
1) that BKL canted rail with a couple of standard 30mm mounts on top. Not adjustable, but will almost certainly put you "in the ball park", as the Yanks say (if you can find anywhere to buy it).
2) the Sportsmatch 2-piece 30mms, which as they *BOTH* tilt, it should be possible to retain perfect alignment between front and rear ring.