Regarding the length of NR seat posts...
yes, that info I believe to be correct, the longest NR/SR seatposts were "only" 180 mm.
For me at 6'-8", I feel your pain ( i.e the cramps which come with too low saddle position). My solution was to take two NR seatposts and have a machinist pal cut-off the top of one and lathe it to very snugly fit into the bottom of the other (I recall heating up the female and freezing the male end - hey now!).
He then welded the two together. Biggest trick is to be careful on the alignment to make the two come out straight. The weld was then lathed smooth, and since I use a piece of tape to mark my position, the tape ended up covering the "scar". I cut off a few inches off the bottom of the assembly to make it easier to insert into the frame.
Just buy a cheap scratched up seat post in the diameter you need and you are good to go... find your own machinist pal. Perhaps a list member will take a shine to the project...
Best of luck, tall boy
Riccardo "try basketbal, kid" Bulissimo
foggy cold Verdi Nevada