More tricky than it appears under Windows, especially if you’re using a network drive path because it’s a guerilla dev installation. You can’t set the props through the command line, it appears, but you can through Tortoise. The correct field for svn:externals was:

tools file:///J:/repo/tools

Setting this prop on the directory that will be the parent of the ‘tools’ directory worked just fine. Now we only need one tools directory in the repo and the export of the source will be much cleaner.