Thanks guys -- I figured it out. While poking around in the plugin source code, I noticed it references the PluginData directory relative to the "saves/<game>/Ships/VAB" directory (i.e., "../../../../PluginData/subassemblyLoader/subassemblies/"). My saves directory is actually a symbolic link to a directory in Google Drive, which I use to sync saves across computers, and this was tripping it up. Even though the error message said it was failing to save the file in the "/Steam/SteamApps/common/KSP/PluginData" directory, it was really trying to save in a PluginData directory that was a sibling of the symbolic link target, in my Google Drive directory. So I made PluginData another symlink in the same location, and problem solved! Appreciate the help.