Oh, now that's service above and beyond!!!!! And of course, as soon as I tried the search with my suggestion, I realized that it wasn't sufficient, as I have a number of books tagged "keep off kobo", which are explicitly not synced to my e-reader, so adding not tags:"=keep off kobo"
was necessary for what I really wanted. And now it works perfectly! Thank you, very much.
I only just started using the plugin, and have only had an e-reader since September, so asking for a new feature seemed premature, but I figured asking to restrict it to "formats:true" was just a bugfix
Until somebody comes along who keeps books without any formats. I've heard of people doing that.
but if it has no formats, you can't copy it to a device (at least not a Kobo):
calibre, version 0.9.35
WARNING: No suitable formats: Could not upload the following books to the device, as no suitable formats were found. Convert the book(s) to a format supported by your device first.
I only send epubs to my Kobo - but I have pdfs and various other e-formats, so using "formats:true" would still show lots of "books" I don't want on the reader, but the ones that have no associated, TXT, EPUB, RTF, DOC, PDF, MOBI... can't be transferred to a Kobo, at least (I can imagine there might be readers that will let you load the metadata without any 'data').