Lance & Yves, thanks to both for your recent replies.
Two new developments;
(1) I have been reading posts at sevenstring.org, and
(2) I have started drawing full scale mockups to see how they look.
There is one possibly questionable assumption that I have been working with: that the high a (or g) string should be fluorocarbon rather than nylon. I see that Galbraith uses nylon, and that hence there is a tone difference with the lower strings for which he uses fluorocarbon. The smallest diameter useful fluorocarbon string is .47mm; I have tried .405 (edit: it was .435, sorry), which is available but it is too thin to have good tone. Now, Mikfik posted above that he was able to tune a .47 savarez FC string to high A-440 at 63cm, but I have been unable to get this result with seaguar .47 FC. I got it up to an A at 56 cm and up to a G at 60cm, not far short of the breaking point. I will need to redo these experiments again soon. Anyway, these test results are why I have the high a length at 56. I am not so sure about the tone quality of a high a string anyway - this may be the biggest weak point in my whole scheme, and why Plan B is to default to G-lute tuning starting at 60 cm or so.
Yves asked, "Anyway, did you already find a guitar-maker ready to try such a short first string while assuring a musical result ?"
To which I reply, I have a luthier who is a proven adventurer who will probably go for it... but I agree that the high A is the weak point.
At sevenstring.org there are many electric players, (and a few steel-string acoustic players, and almost no nylon strings, other than one Bartolex commercial) who play fanned fret extended range instruments. In that electric guitar zone, there are no harp guitars, but there are fanned fret instruments up to ten strings, and many with eight strings. Most people there who have commented seem to agree that a 3" (7.6mm) fan spread is comfortable for "most players" with the perpendicular fret around the 7th to 9th fret. There are a few who advocate a 4" (10mm) fan spread - these are players who want the usual six strings plus a low B and an even lower F#, the length on the low F# being 28" (71 cm) and the high e shortened to 24" (61cm). It appears that the short end of this spread would also make possible a high a-440 with the "right" string. By contrast, Woodhouse is using only a 1-3/4" (3.5mm) fan spread, from 615 to 650, very conservative by comparison, and it appears that he has reduced it after experiments with larger fans (??). Now, how that which works for electric guitars may work when applied to classical guitars remains to be proven.
In general, the electric players at SSO say that it is "no problem" to adapt to playing on the fan fretting. They also say, generally, that intonation problems are actually reduced by the graduated scale, so that all string thicknesses are not forced into the same length and thereby reducing differences in the effects of stretch and inharmonicity (concepts which I do not entirely claim to understand, and throw around like word salad to show my erudition
). (With regard to this, I would propose as with my other guitars a wide bone on the saddle, 3mm or more to allow possible increases in setback for intonation adjustment.)
Not wanting to be accused of not thinking far enough outside the box, as a projection of a thought experiment I drew up a full-scale diagram of my most radical possible design (excluding the "virtual 12 string" as not realizable, of course) which is a 9 string (some players have electric fanned fret 9 strings at sso) with the high a-440 at 56cm and a low B and then a low F# at 71cm. This is a 6" (15 cm) fan spread, of which 2" (5 cm) is expressed at the nut end (because I set the parallel fret at the 7th fret, or 1/3 for _easy calculation_, and only for that reason.) With the strings near the nut spaced at 9 mm, the total fingerboard width is around 80mm (don't have the drawing with me, I'm at an I-net cafe) which I believe is too much for my index finger to get across at the fanned first fret, so the string spacing would have to be reduced to 8mm probably even if the rest of the design were workable. The extreme pitch range seems to me to be unlikely to work with an acoustic box; with electric guitars the signal is not acoustic so it's no issue. However, the 6" fan spread itself does not seem entirely unworkable... when the parallel fret is #7, the amount of spread expressed at the nut end is only 1/3 of the total, and the differences between my drawings do not appear that extreme. I have not yet cut them out and pasted them onto cardboard, which I will do soon so that I can hold them like a guitar and ... meditate.
The other completed drawing I have is for an 8-string high a-440 at 56cm and a low B at 66cm, a fan spread of 10 cm or 4 inches. As I said, some electric players say that this much spread works fine for them. The 1/3 of the spread expressed at the nut end is only 33mm, looks easy compared to the 6" spread!
These drawings take me some time. Anyway, I am getting some concrete ideas. Next up on the table will be the g-lute model, with the high g at 60 and the low A at 67, a 7cm spread or 2-3/4", of which about 2.3 cm would be at the nut end. Compared to the other possible designs this is starting to look like no problem at all!
Now, it is time for me to go and watch the sunset, to prove that I am not a computer addict!
over and out.