I'm not sure how likely this will happen. But a lot of "special" items break when put into backpacks. So the only option is the blacklist the item in general.

Would it be possible to add an extra line to the blacklist for a skinid?

0 blocks the item in general. And a skin will block that specific skinned item so it can't break.

It would require a config wipe. But you could just have your demo items with the "skinid": 0 line user it so users can just copy paste.