There's a big square in the middle of Lorient where there is a lot of busking going on during the festival.
I'm sure if you just started busking your routine in the square it would go down a treat.
There's also a lot of busking in Quimper.
Also, as Philippa says, there's a Fest Noz going on in almost every night. I'm sure the organisers would only be too happy to put on an extra act - especially if you could include something that the audience can join in.
You could also put a wee ad in the local journal Ouest France. It's a good paper for finding out where the gigs, concerts and Fest Noz's are taking place.
Here their site http://www.ouest-france.fr/
Whatever happens you'll have a ball. It's a great place