I'm not exactly sure if harry.lu is a tool offered by e621 itself, or if it's an independently-hosted thing, but if it runs off the original IQDB code, then it should have an index.xml that my booru can interface with directly. If it does, where would I link my booru's "similar images" tool to?

If I were you I'd ask yesthebest about solutions for implimentation, since he's the one mentioned on the site and everything.