You are absoloutely right! The Piedmontese army did mobilise and fight against the French in 1815.
The Austrian Commander Frimont commanded an Austro-Italian army of between 50 and 100,000 men. This army contained a corps of between 12 and 15,000 men under General Latour. There was an additional force under the command of the Piedmontese commander D'Osasco (Full name possibly Policarpo Cacherano D'Osasco -though this still needs verifying!)
Just out of interset I may point out that so far I have been unable to find any mention of a 20 odd thousand strong Army of Naples in 1815 commanded by a General Onasco that is mentioned in a few recent books. What I have found out is that the Piedmontese had a force under D'Osasco (as I've mentioned) and that Bianchi commanded an Army of Naples (part of Frimont's army) which was of a strength of about 20 odd thousand. It appears to me that the latter two have been confused with one an other and the end result was Onasco's Army of Naples.
Back to the point! Sadly information in English about the Piedmontese in 1815 is scarce. An English gentleman called Colin Allen was hoping to produce some material in First Empire magazine, after writing his excellent Neapolitan Uniforms articles MANY years ago. But, I don't think he got 'round to it - though I could be wrong!
I contacted Ludovic Isnard after reading his excellent Piedmontese Army article on this site and noticing that the Piedmontese in 1815 were not mentioned but he assured me that that some information would be posted in due time.
Regarding flags there is an excellent website which I have given a link to below and a few that discuss uniforms and the Carabinieri. No detailed OOB has materialised as of yet but I'm hoping to track some down from Italy soon.
Here are some links you may find useful:
Or in German if you prefer:
English books worth mentioning are Siborne's Waterloo book (which is on Google books) which will tell you a little about the Piedmontese actions near the back of it as will Digby Smiths Napoleonic Wars Data Book.
Hope this is helpful,