It's currently not possible to import (solvable) SegWit addresses (P2SH and native) using `importmulti`. I believe that's a shortcoming we should address before release.

My suggestion would be for P2WPKH (P2SH and native) to be supported without any extra arguments. This means that a P2WPKH address/script with corresponding private or public key should work. For P2SH it means checking if the supplied P2SH address perhaps matches the supplied public key or private key when converted to P2WPKH first.

To support P2WSH, I think we need an extra field "witnessScript". When supplied, it would either be required to match a P2WSH address/script directly, or if the address is P2SH, indirectly.
{
    std::shared_ptr<CWallet> const wallet = GetWalletForJSONRPCRequest(request);
    CWallet* const pwallet = wallet.get();
    if (!EnsureWalletIsAvailable(pwallet, request.fHelp)) {
        return NullUniValue;
    }
    if (request.fHelp || request.params.size() < 1 || request.params.size() > 2)
        throw std::runtime_error(
            "lockunspent unlock ([{\"txid\":\"txid\",\"vout\":n},...])\n"
            "\nUpdates list of temporarily unspendable outputs.\n"
            "Temporarily lock (unlock=false) or unlock (unlock=true) specified transaction outputs.\n"
            "If no transaction outputs are specified when unlocking then all current locked transaction outputs are unlocked.\n"
            "A locked transaction output will not be chosen by automatic coin selection, when spending bitcoins.\n"
            "Locks are stored in memory only. Nodes start with zero locked outputs, and the locked output list\n"
            "is always cleared (by virtue of process exit) when a node stops or fails.\n"
            "Also see the listunspent call\n"
            "\nArguments:\n"
            "1. unlock            (boolean, required) Whether to unlock (true) or lock (false) the specified transactions\n"
            "2. \"transactions\"  (string, optional) A json array of objects. Each object the txid (string) vout (numeric)\n"
            "     [           (json array of json objects)\n"
            "       {\n"