I had to apply to the FAA 6 weeks prior to visiting and they checked with the NZ CAA, then I had to nominate an FAA Flt. Service Station where I would personally present myself, with I.D. to collect my temporary validation, which would then only be valid for 6 weeks after issue.
This sounds exactly like the procedure you need to go through when you first apply for a "piggyback" FAA PPL. Once you've got that piggyback PPL, it is supposed to stay valid for life, provided that the license it is based on is valid, and that you've done a BFR not more than two years ago. I have never heard about the six weeks period in this context.
It also doesn't seem like the page describing the process has been changed lately.
http://www.faa.gov/licenses_certific..._verification/
There's also nothing in the FAR 61.75 that mentiones a limitation to six weeks:
http://www.airweb.faa.gov/REGULATORY...D?OpenDocument
You're talking about an FAA CPL "piggyback". I'm not familiar with that procedure but if you're only going to fly for pleasure in the US, it looks like nothing has changed and it is fairly painless to obtain an FAA PPL based on your NZ CPL.