My debug attempt was with the unzipped gmapsupp.img contained in gmapsupp_francia.zip. In its 15 January version, it was about 315Mo, and was ok for direct use in the gps, but made sendmap20 fail.
Today, I downloaded a new gmapsupp_francia.zip (dated 19 january) , and it appears that its gmapsupp.img is only 255Mo, and seems ok with sendmap20. Strange isn’t it?
Then I succeeded to combine the gmapsupp_francia.zip with one of my custom img and typ file, and it worked.