My club did a barrel project that we aged for 15 months in the barrel and it was soured with Roselare. What I did was took a 2nd generation yeast cake from one of my 5 gallon batch's and used that. Most of the people in the club did primary ferementation clean with an Abbey Ale, and a few of us fermented in Primary with Roseleare or other Bacteria blends. When it came time to transfering into the barrel we put the beer in the barrel (after sampling them all) and then dumped in the yeast cakes from the 2-3 mixed fermentation batch's as well as the large yeast cake I had from my Flanders. That's my recommendation. Think of each batch that is fermented with Brett/bacteria as a starter. Re-pitching bugs 2nd and 3rd generation will produce better product then simply pitching 1st generation roselare (in my experience). 2nd and 3rd generations will be more aggressive for souring your beer. This is critical when most of your beer is primary fermented clean.
We just bottled our beer after 15 months in the barrel, and it is absolutely amazing.
Reach out to me if you have any more questions as we successfully created a barrel aged sour that is as good as anything on the market! We did a RR-Temptation clone and it's spot on!
Maybe these links will help..
give a call with questions