Seymour, You usage of the even/odd R0/R1 pair appears to be reversed (unless you intended to zero some memory at whatever location L'DEST resolves to ... not to worry, it happens to dyslexic programmers like me all the time).
I don't know of any means by which you can split the definition of an immediate field such as you suggest, and I'm not sure I'd welcome that sort of complexity for what is otherwise a relatively simple concept: immediacy.
