There are many topics in this post and I will take them in random order:
Taylor's chooses not to make a
Colheita because the want to focus on Vintage Port. I have spoken to the Managing Director about this, as recently at June 2005, while asking if it was a direction that they might go in the future. After our discussion, I believe the only way that Taylor would ever head this way (which I don't think they ever will) would be to purchase old stocks of Vintage Dated Tawny Port from another producer. So don't count on this any decade soon.
But, since the Fladgate Partnership also owns Fonseca, Croft and Delaforce ... it would not surprise me at all, to see Delaforce which historically has produced some intriguing wood-aged Ports ... to produce a
Colheita in the future.
Niepoort - the reason for what Frederick calls "freshness" in the Niepoort styles, is almost a cliche. Every article ever written on Niepoort mentions the freshness, because we all know that Dirk loves his Ports to seem fresh.

As to the reality of things, there IS a freshness to his
Colheita and that comes from the use of ancient glass demijohns. See my article and photos on Niepoort Colheitas
http://www.fortheloveofport.com/articles/a_colheita.htm
I do think that there are other companies that also make some excellent Colheitas and off the top of my head, Burmester, Quinta do Noval, Kopke, Quinta do Portal, Ferreira, Krohn and Rocha all come to the fore.
Now to the crux of the matter that started this thread:
The IVDP regulation does not get into enough detail to say that
topping off with a younger wine, is "illegal." In fact, I know that this practice does go on. It is not done to make the wines more youthful although, that may be the outsome of the practice of
topping off the evaporated wine.
I have requested more specific information directly from Portugal to get clarity on this topic and we can avoid innuendo once I receive word back.