In certain rare cases it would be nice to ask some IPv6 DHT nodes to always send us IPv4 nodes they know.

For example, there's a way to bootstrap internal DHT for torrent clients inside Yggdrasil (or just any private overlay network) using existing trackers (in Russian, but the idea is obvious):
https://yggwiki.cc/yggdrasil:bittorrent:dht

Some of those clients are multi-homed, and can provide us with public IPv4 nodes for DHT searches. It might sometimes be beneficial to learn about nodes that far away users have in their table instead of nodes that currently surround you. Sending "want"=["n4","n6"], as defined in BEP 32, and using data from both searches might help with that.

Because it is of no use in the public network, it should certainly be off by default.

If we get a little crazier, we can even imagine bootstrapping some other overlay IPv4 network, and relaying IPv6 peers using the same trick. If we get a little crazier, we can even imagine an extension to I2P (["n4","n6","i2p"]). If we get a little crazier, we can even imagine metadata transfer and storage via DHT, and background cross-pollination via some kind of sample_infohashes bulk transfer for node ID and nearby area. If we get a little crazier, they will finally put us in the madhouse.