Offset - literally

Offsetting from a spline or polyline freeform curve, inside the curve - no prob till the specified offset gets bigger than the local radius of the curve - when the offset line would have to cross itself. Then Acad just leaves out the overlapping bit. I want to see what happens when the offset still happens literally, with interesting curve reversals etc.

If Acad won't do it, anyone know of software that would , perhaps as a toggled option?

All I can think of is that you'll have to mirror your spline?

I'm pretty sure AutoCAD won't do it for the reason you've mentioned above.