Considering the French say "soixante-dix" for "seventy", how possible is it to say "soixante-vingt" for "eighty" and "soixante-trente" for "ninety" as the logic behind is the same. You will most probably mention about conventions but:
Is it ever used that way by any French speaking society/person?
No contemporary usage but soixante-vingt(s) would likely have been understood to mean 1200 (i.e. 60 x 20) in the Middle-Ages and the next few centuries, when the vigesimal system was still customary.
Would a francophone understand what I mean by "soixante-vingt" ?
No, unless perhaps you recite numbers: soixante-dix-huit, soixante-dix-neuf, soixante-vingt...
What would be their reaction if I ever say "soixante-vingt" or "soixante-trente" ?
A good laugh.
How about "trois-vingt-dix" ?
That one is regular and was used in the past. Some rare occurrences can even be found as late as the eighteenth century but would no more be understood nowadays.
That reads:
Le trente octobre mille sept cents soixante quatre décéda Jean de Hauret tisseran de la marque de Candaloup agé d'environ trois vingts dix ans. Son corps a été ensevely le trente du dict mois dans le cimetière de...
Source: Geneanet
Note that a similar construction used to exist in English too:
John Horsfall was indebted in the sum of three scores and fifteen pounds to one Christopher Mitchell late of Chastleton. (1577), The YorkShire County Magazine, 1891
See also:
1/ It is never used, that is the first time I hear about this idea.
2/ You can't do that, not at all: people will not even guess what you are saying. If you are speaking with Belgian people, then, as they sometimes use "octante" instead of the usual "quatre-vingt", you might say "octante", but in France, only a very well educated person would guess right away that you mean "eighty".
3/ The reaction can be an immediate interruption of your sentence and a request for an explanation, or it might be nothing on the spot because the person you are talking to is not yet quite sure whether something is amiss or not, and is thinking that it is better to let sink in what you are saying. This is typical of all verbal exchanges in which improper terms are used.
4/ This form is foreign to the way of formulating the first numbers in French, just as the other forms you are talking about.
Note ajoutée à la suite de commentaires
(Wikipédia) Si, « octante » est employé parfois en Belgique aux côtés de « quatre-vingts ». En revanche, huitante est inemployé hors de Suisse.
(Académie) Il est à noter pourtant que les mots septante, octante, nonante figurent dans toutes les éditions du Dictionnaire de l’Académie française. Encore conseillés par les Instructions officielles de 1945 pour faciliter l’apprentissage du calcul, ils restent connus dans l’usage parlé de nombreuses régions de l’Est et du Midi de la France, ainsi qu’en Acadie. Ils sont officiels en Belgique et en Suisse (sauf, cependant, octante, qui a été supplanté par quatre-vingts et huitante — en Suisse — tant dans l’usage courant que dans l’enseignement ou les textes administratifs). Rien n’interdit de les employer, mais par rapport à l’usage courant en France, ils sont perçus comme régionaux ou vieillis.
