Alas! This version returns the damaged dialogue again; once again after I removed the xattributes myself it ran perfectly. My friend agrees you'll need to code sign it again if you've changed anything.
Damn. I hoped re-exporting the original that worked and adding the missing _CodeSignature folder from the Mac export would be a workaround, but if it needs a new signature each time then that's not an option. And I don't have access to the borrowed Mac so it could be a while before I can find a proper fix.