I'm really jealous of the pallet planter- I've wanted to do one of those for a long time..
Yes, I did drill a hole in the bottom of the shell which was a lot harder than I thought it was going to be. I figured a masonry bit would go right through it- but no.. when that didn't work (which is crazy, becasue its basically what limestone is made of)- I make a small starter hole with a 1/8 tile bit, then enlarged it with the 3/8 masonry bit. You have to watch where you put the holes, because the empty shell balances on two points- the long tip and the widest part of the spiral, but once its full of dirt, the center of gravity shifts. The long tip comes off the surface, and it sits upright on only a single point at the widest portion. when you water it and the dirt gets heavy it shifts even farther, so I'm not sure the hole I drilled at the balance point when its dry and empty will be sufficient for drainage.. there's so little soil that I think a wet spot will be fine over time. I'll let you know how it goes.
and if you have a flaw in the top like I did, where I put a graptopetulum & additional cuttings, you have to make sure you fill and shake, fill and shake, then fill and shake 10 more times to insure you have a substrate that won't just fall further down the spiral over time