Hi, the dumpwallet command throws the private keys and a xprv.
Can I use it to derive all future addresses from the wallet?
So long as you do not change your password or set one. If you do, the xprv will change.
I'm a bit out of sync, I didn't know the reference client implemented HD keys.
Only in 0.13.0+
If so, could anyone tell me the derivation path used?
It uses the standard paths as defined by BIP 32. That is m/0/0/k for addresses you get from the wallet, and m/0/1/k for change addresses.