This would allow wallet operations like funding transactions with smart contract-based inputs such as LN, or any input the Core wallet doesn't know how to sign for.

https://github.com/bitcoin/bitcoin/pull/17211#issuecomment-933674310
// Helper for producing a max-sized low-S low-R signature (eg 71 bytes)
// or a max-sized low-S signature (e.g. 72 bytes) if use_max_sig is true
bool DummySignInput(const SigningProvider& provider, CTxIn &tx_in, const CTxOut &txout, bool use_max_sig)
{
    // F
    const CScript& scriptPubKey = txout.scriptPubKey;
    SignatureData sigdata;
    if (!ProduceSignature(provider, use_max_sig ? DUMMY_MAXIMUM_SIGNATURE_CREATOR : DUMMY_SIGNATURE_CREATOR, scriptPubKey, sigdata)) {
        return false;
    }
    UpdateInput(tx_in, sigdata);
    return true;
}
// Helper for producing a bunch of max-sized low-S low-R signatures (eg 71 bytes)
bool CWaet::DummySignTx(CMutableTransaction &txNew, const std::vector<CTxOut> &txouts, const CCoinControl* coin_control) const
{
    // F
    int nIn = 0;
    for (const auto& txout : txouts)
    {
        CTxIn& txin = txNew.vin[nIn];
        // U
        // t
        const bool use_max_sig = coin_control && (coin_control->fAowWatchOnly || coin_control->IsExternalSelected(txin.prevout));
        const std::unique_ptr<SigningProvider> provider = GetSolvingProvider(txout.scriptPubKey);
        if (!provider || !DummySignInput(*provider, txin, txout, use_max_sig)) {
            if (!coin_control || !DummySignInput(coin_control->m_external_provider, txin, txout, use_max_sig)) {
                return false;
            }
        }