I believe Schneider exported mountain guns to Serbia so if the Mle 08 was one of the types sold to Serbia it would have seen action in WW1. There were a large number of different types of French mountain guns built before WW1 - I've never seen an authoritative text which sorts out the various types.
The only thing definite (and then, not really definite) I've found so far is a mention in Artillery: An Illustrated History of Its Impact, by Jeff Kinard. He states it was adopted by the French military in 1908, and also that it was fielded in WW1.
An "authoritative text" on Schneider (and also Saint-Chamond) mountains guns built betxeen 1902 and 1919 exists: It is a paper of Capitaine Mortureux in french review "Revue d'Artillerie" (edited in five articles from april to september 1922, volumes 89 and 90), the papers were also edited in a book with photographs. This paper give a summary of all typs of Schneider mountains guns(and also Saint-Chamond mountains guns) built between 1902 and 1919. They are 19 different Schneider mountains guns models built for 16 countries in the world (some countries have two or three different models). The year "1908" is not very precise because Schneider mountains guns are knowm on a mixture of letters and figures (MPD or MPC 2 and so on). In 1908, Schneider built mountains guns for Montenegro, Serbia and Peru. Each country had a special year of adoption for the guns buyed. A SPECIAL ANNOTATION: in many "web sites" (foreign and also french!), one read that the french "canon de 65 mm de montagne modèle 1906" is Schneider made.It is totally wrong and stupid, the "65 mm de montagne modèle 1906" guns were made only in french States Works (Atelier de Fabrication de Bourges), this model was adopted in France in 1908 and in Serbia and Greece during the 1914-1918 war...but it is NOT a Schneider Gun! But the Serbia and Greece had adopted others Schneider mountains guns BEFORE the war. Yours sincerely, Guy François.
On official notes of Schneider, I read : -built for Serbia: 70 mm M.D 2 Mountain Gun, 36 guns made ("canon de montagne léger"), 75 mm M.P.D 5 Mountain Gun ("canon de montagne puissant"). -built for Montenegro: 75 mm M.P.D 5 Mountain Gun. Yours sincerely, Guy François.
Exactly 75 mm M.P.D. 5 was only one of options for Serbians mountain artillery after Balcans wars in 1913 for great orders mountains artillery. Serbia had a plan to the third division in an artillery regiment had the mountain guns.
I'm not sure if your model has any relation with the subject. Wasn't 70 mm mountain gun M1908 a derivative of Greek colonel Danglis design? Such a gun was license producted in Spain: http://amonio.es/canon_7016.htm