Unbundled Info.plist on 10.5 Leopard

We recently added a non-privileged, unbundled (stand-alone) helper tool to our main application bundle. At first it showed up in the Dock, but then we added LSUIElement to that application's Info.plist. Naturally, we embed that plist with -sectcreate __TEXT __info_plist Info_plist_file.
However, 10.5 did not respect this setting. It insisted on displaying a duplicate Dock tile for the main app -- same name and icon. We tried several variations, but the only thing we found that works on 10.5 is to make the helper application a Cocoa app bundle and include its Info.plist there. Only then is the LSUIElement preference respected.