It is not currently supported.
However, @stgraber showed a workaround in one of his live sessions https://youtu.be/XIqVUQk4EMs?t=4156, which you can replicate with the following.
incus launch docker:valkey/valkey some-valkey -c raw.lxc=lxc.execute.cmd="valkey-server --save 60 1 --loglevel warning"