> re "let it fail" though, I hope that doesn't mean the "there's a bug in NetLogo" dialog ensues

You just get the regular "No such file or directory" dialog.

> re: checking validity, File.exists, perhaps trapping exceptions?

Well, `File.exists` would check for existence which, as per my bullet points, I do not want to be a criteria for inclusion in the submenu (though one may argue that it should). What I was looking for was a way to check for filename validity without checking for its existence.