Thanks! I didn't know about the plugins folder.
If anyone else wants to make this fix, add the following lines to ChopTrees.properties (in the plugins\ChopTrees folder):
#Bronze Axe
mat = 30199
#Chainsaw
mat = 30233
Obviously, you can remove all the "id = " entries -- someone on the Tekkit team guessed wrong there. If you want to add Ruby, Emerald and Sapphire axes, those IDs are 1284, 1285 and 1286. (I run my server without RedPowerWorld/RedPowerMachines -- the giant rubberwood trees are way too common, and way too big, and they ruin both the IC2 rubber economy and the landscape -- so no gemstone axes for me!)