Kevin,
Even with the -bs option, MacRuby app might still not be able to load the bridge support preview properly on 10.6. Here's what I do for Gmail Notifr:
dir_path = NSBundle.mainBundle.resourcePath.fileSystemRepresentation
load_bridge_support_file "#{dir_path}/BridgeSupport/Security.bridgesupport" if NSAppKitVersionNumber < 1138
1138 is the version number for 10.7, on which it doesn't have problem.
The above lines are put in rb_main.rb file. You can see the full file if interested:
Hope this works for you.
Cheers,
James