Very interesting! It's like the best of the fragment-pre-encrypt world (everything appears as single packet 5 tuples to middleboxes) and fragment-post-encrypt world (transported packet data remains untouched) debate seen on IPsec deployments.

Like you mention you could do this under QUIC but then you'd be hamstrung to some of the design mandates such as encryption. This is way better as it's just datagrams doing your one goal - hiding that you're transporting fragments.