restore wallet using hd seed before 0.17.0
I cannot find hdseed in dumped wallet file, so I don't know hd seed to backup

It used to be called hdmaster rather than hdseed.


creating new wallets is done by starting bitcoind and passing -wallet option, but I don't see a way to specify hd seed when creating a new wallet

Is that even possible without upgrading?

No. sethdseed was added in 0.17. Before that, there was no way to restore a seed to a new wallet in Core.