I'm truly stumped at this one. The only thing I have left is maybe the expansion tank has developed a crack, being old and heating up and cooling process of the coolant. This is a common problem on the E38's but I don't recall hearing about it on the E32's. Take the expansion tank out and check for cracks on that. If the tubes and pipe work is fine, as you said it was, then the only thing I can think of would be a crack somewhere you can't see, which would explain why coolant just leaks out as soon as you dump it in. If the tank is fine, I'm miffed as I can't think of any other possibilities.