The user of-course can already implicitly work out this information from comparing the fee-rate with the actual fee.

I think that it would be useful for people not wanting to do this arithmetic in their head.

{
public:
    CAmount nValue;
    CScript scriptPubKey;
    CTxOut()
    {
        SetNull();
    }
    CTxOut(const CAmount& nValueIn, CScript scriptPubKeyIn);
    ADD_SERIALIZE_METHODS;
    template <typename Stream, typename Operation>
    inline void SerializationOp(Stream& s, Operation ser_action, int nType, int nVersion) {
        READWRITE(nValue);
        READWRITE(*(CScriptBase*)(&scriptPubKey));
    }
    void SetNull()
    {
        nValue = -1;
        scriptPubKey.clear();
    }
    bool IsNull() const
    {
        return (nValue == -1);
    }
    uint256 GetHash() const;
    CAmount GetDustThreshold(const CFeeRate &minRelayTxFee) const