I hadn't come across Joseph Guy Ropartz before - his dates are 1864-1955, so he was in the generation before Villa-Lobos. The very informative Wikipedia article provide the following information:
Born in Brittany, Ropartz studied in Paris with such important teachers as Jules Massenet and César Franck. From 1919 to 1929 he was busy in Alsace, as the Director of the Strasbourg Academy, and of the Philharmonic Orchestra. In 1929 he retired to Brittany, and died there in 1955, four years before Villa-Lobos.
This short resume of Ropartz's life shows that it was very unlikely that Villa-Lobos's connection to him was personal (as so many of his connections were - think of his close relationships with Edgard Varese, Olivier Messiaen, Florent Schmitt). The newly-discovered orchestrations were dated 1921, and Villa didn't arrive in Paris until 1923. In spite of the difference in their ages and their musical interests, though, there is a connection between these two composers. A French blog post about the newly-found scores calls Ropartz "son homologue français," Villa's French counterpart.
The Wikipedia article points to the probable reason that Villa chose this composer to adapt:
Ropartz wrote that he was the son of a country "where the goblins populate the moor and dance by the moony nights around the menhirs; where the fairies and the enchanters - Viviane and Merlin - have as a field the forest of Brocéliande; where the spirits of the unburied dead appear all white above the waters of the Bay of the Departed." This interest in the world of folklore was shared by the composer of Amazonas and Uirapuru (1917), both of which are about the same fairy-world creatures of the forest that interested Ropartz. I very recently blogged about the mythological forest being that inhabits the First String Quartet, from the same period (1915).
The Wikipedia article includes this quote about Robartz by Rene Dumesnil in Le Monde. It could have been written about Villa-Lobos as well:
"There is with Ropartz a science of folklore and its proper use, which one admires; but more often than the direct use of popular motifs it is an inspiration drawn from the same soil which nourishes the work, like sap in trees."I'm listening to Ropartz's First Symphony "On a Breton Chorale" on a Timpani recording with the Nancy Symphony, conducted by Sebastien Lang-Lessing (on the Naxos Music Library). This music sounds as connected to the folkloric world as any of Villa's works. "I am Folklore", Villa-Lobos famously said, and these motifs were completely internalized in his music, permeating his greatest works (any of the Bachianas Brasileiras, for example), whether or not they have a program.
An additional musical link between Ropartz and the earlier symphonic works of Villa-Lobos may exist because of their shared adherence to the compositional teachings of Vincent d'Indy, though I'm not well enough versed in musical theory to pull out those particular threads myself.
I predict that when more information is released about these new scores, it will turn out that the Ropartz works in question have folkloric themes.