$ubaru Oz is happy to sell you -- a complete new enclosure, for $250+ (the wreckers wanted $110 for it). "No, you can't take the glass lens off, it's glued on." Yeah, sure...
A quick 15 minute trip in the oven (160°C) and some careful prying with a putty knife later, and I had the 'unseparatable' lens separated without any problems. Superglue took care of the crater; it's quite nice that it corrected all the nasty optical effects of the crater and only a faint outline of the chip remains visible (3 o'clock above the E sign).
Some gasket sealant, another oven excursion and a bit of careful pressure made the lens-enclosure joint solid again. Net cost: under an hour of fiddling, and I didn't have to throw away the mostly sound lamp.
