Your idea about grafting the clear outer lens from an altezza tail onto an oem fixture is the only way I see. The two would need to have the exact shape/dimensions or it won't graft cleanly. Once uncovered, the insides of the fixture aren't all that pretty. You may need to paint that area or something. I have pictures of my B5 tails with the lenses removed for an LED project I did.
http://www.passatworld.com/forums/volkswagen-passat-b5-discussion/353923-led-tails-b5.html
Taking the lenses off these tails and grafting them back on isn't as easy as headlight fixtures. With headlights, the fixture body and lens are glued together with a substance that goes soft when heated. People do a slow warming in an oven then pull them apart. With these tails the fixture and lens are permanently glued together with what appears to be a solvent style process. To separate, you need to carefully cut them apart along a seam that makes several bends, curves, corners. It took me a while of careful concentration to do mine and I'd say mine turned out about 80% quality. If I did it again, they'd be a bit better/cleaner.