RE: Private use of non-RFC1918 IP space

If you get an address reservation from a registry, then you could certainly use that space in a way that doesn't entail globally-reachable routing. In fact, IIRC one of the RFCs explicitly mentions this possibility in the event that overlapping private use address space usage makes interconnection between two networks otherwise unworkable.

However, as everyone else had assumed you might be doing, "borrowing" unassigned or assigned-to-someone-else (for which all "unassigned" can eventually be expected to devolve into) is unwise.

– S