I know this is all "multicast" space, but considering classless routing
and CIDR, shouldn't this space be usable and able to be manipulated ?
Nope. Its still special. Once you turn on multicast support,
you get to "diddle" in that space although not directly.
See the nanog archives from late 1996/early 1997 for exaustive
discussions on what were special use addresses and what became
special use. Look for "Martian" in the subject thread.