Historic Rego for the 260z help please

[Captiva Fan]

Is it easy enough to go back to full rego or even possible at all?

Worst-case scenario would be an invalidation of an engineer's certificate (ie. they usually expire when your car goes over 3 months out of rego); if you don't need one, then worst-case would just be the need to pass a blue-slip (not difficult).
Would be interested to hear what Folk Who Know say about the engineer's cert, and whether maintaining historic rego counts as "not letting rego expire"?