For our Brett Pros...

A bunch of us up here in the Pac NW just blended several batches into a Pinot Noir barrel and dumped in some brett and lacto. All batches were previously fermented before blending. We're a little shy on volume into the barrel so I was going to do up another batch. Should I let this batch ferment out for a few weeks then blend, or will the bugs benefit from having some fresh sugars added?

Our club did a pinot barrel flanders not long ago, and we ended up having to brew a couple of extra batches to top up the barrel over the 18 months it was in there. The angels must have really liked our brew:rockin:
Anyway, go ahead and brew another batch, it'll be fine with fermenting it out then blending. There's plenty of sugars for the Brett. et al. to chew on already.

A bunch of my friends and I are going to be putting Flanders Red into a barrel next weekend. We are planning on adding 5 gallons of unfermented wort along with 55 gallons that has already fermented, just to make sure the brett/bacteria starts out happy. When I am normally doing sour beers I like to add the funk right at the start, the more sugar it gets to work through the happier I am with the resulting beer.

I agree though for top-ups over the next 12-18 months I would go with already fermented wort since it will be less likely to dramatically shift the conditions in the barrel. I know Lost Abbey keeps a barrel of their Mexican lager around to top-up all of their other barrel aged sour ales.