Browsing the Mustradem.com site I see they have opened their "
Stage d'improvisation musicale" course subscription early this year. It has run 29 years now, but announced as late as december in the past!
As ever - "all instruments" - but non musical impro may take a lesser role (title has changed). You'll need at least 2½ rows or a halftone accordéon but IME a D/G has not disadvantaged me compared to a G/C. In fact the naturally #4ths in its main row scales sometimes came in handy!
Stage Impro teaches what you do "after" you have stated the theme of a tune, be that in (re)arrangement or an improvised line. The course does
not teach instument, melody, repertoire or even jazz, though the latter is always in the background. It's all in French, (though we "carried" an Estonian who had English in 2013 and occasional Italians before that)
Notes are thus in Sol-Fa notation, and chords notated as A7 Bb
dim C#m7 etc. English course notes are available, from this year and offer a glossary of French v English terms
http://stagemydriase.jimdo.com/stage-d-improvisation-musicale/Next year's course/
stage will run
Sun 21- Sat 27 February, and is again in the summer school facility at Bourgoin-Jallieu - a small town on the Lyon-Grenoble line. I TGV'd it (under 5 hours!) to Lyon last year and transfer was easy. They then pick you up from the station. I may extend to a second week this this time as I am owed some 1:1 coaching, but if so could help someone with a lift one way or the other.
Please pass this on to those playing other instrumets. [Wine buffs might also note that the
Salon des Vins festival at nearby Tain l'Hermitage … starts the day this one finishes]