Yes, definitely used mules for the walk out. I have done it previously carrying all my own gear and walking in plastic boots but that was really unpleasant! I'm pretty sure we arranged it in advance - not sure they can take money at BC? They have a system of using tags which works pretty well. I think I showed my receipt to the people at BC and they tagged the bags, gave me the stub and I matched them up back at Puenta del Inca. There would be arrieros at BC but they operate quite independently so it's not easy to arrange things with them. Last time, we packed our two duffels at BC, put them by the BC tent with tags on them, and left around 8am. Sometime after that the mules picked them up - around 930-1000 - and we saw our bags trot past us about half-way out.
Campo Base also has an office in town, along the main pedestrian mall (Sarmiento?) but they no longer do individual unguided logistics, only their own guided groups. I had used them in 2005, they were OK, not great. Grajales is better. I can't find the address for the Grajales office, but it's in one of the streets running perpendicular to the main pedestrian mall. It looks like quite a big travel agency type place from the outside, for tours etc, but they just took us to a desk up the back for the mules.