可以实现0 ETH转账时，返还指定代币的功能

是的，可以通过智能合约实现0 ETH转账时，返还指定代币的功能。合约需要先进行授权才能转账代币。以下是一个简单的合约代码：

pragma solidity ^0.8.0;

interface IERC20 {
    function transferFrom(address sender, address recipient, uint256 amount) external returns (bool);
    function balanceOf(address account) external view returns (uint256);
    function allowance(address owner, address spender) external view returns (uint256);
}

contract TokenRefund {
    address public tokenAddress;
    IERC20 public token;

    constructor(address _tokenAddress) {
        tokenAddress = _tokenAddress;
        token = IERC20(_tokenAddress);
    }

    function refundToken() public {
        uint256 allowance = token.allowance(msg.sender, address(this));
        require(allowance > 0, "TokenRefund: Token allowance must be greater than 0");
        uint256 balance = token.balanceOf(msg.sender);
        require(balance == 0, "TokenRefund: ETH balance must be 0");
        token.transferFrom(msg.sender, address(this), allowance);
        token.transfer(msg.sender, allowance);
    }
}

在部署合约时，需要传入代币的合约地址作为参数。之后，用户可以调用 `refundToken` 函数，来触发返还代币的操作。如果用户的代币余额为0，且已经授权给合约转账，那么合约会将用户的代币转入合约地址，并将相同数量的代币转回给用户。