Yet another thing ipv6 solves. Yeah you can do the officially supported fdXX:XXXX:XXXX:... but you can also just pick something like 1::1 and it's unlikely to conflict with the current global range.

If you're gonna do that, though, it's better if you use fd00:... or one of the other assigned ranges so it's still in the standard range. OSes use this as a heuristic for source address selection.