Extend the Unity Inspector to allow entering a "long" value as a hex string.

HexIntDrawer.cs

using System;
using UnityEngine;
using UnityEditor;

// Create the "HexInt" Attribute that can be applied to a property.
// Optionally, you can specify the number of digits to "zero pad" it.
public sealed class HexIntAttribute : PropertyAttribute
{
    public int digits;
    public string FormatString
    {
        get
        {
            if (digits == 0)
            {
                return "X";
            }
            else
            {
                return string.Format("X{0}", digits);
            }
        }
    }

    public HexIntAttribute(int digits)
    {
        if (digits < 0)
            throw new ArgumentOutOfRangeException("Digits cannot be negative");
        this.digits = digits;
    }

    public HexIntAttribute() : this(0)
    {
    }
}


// The following class should be placed in a script file at the path "Assets/Editor"

// Here is where you create the custom PropertyDrawer. The magic happens in the
// OnGUI method where we create a TextField in the inspector and set it's value
// to the SerializedProperty's value as a long, read it back as a string and
// try to parse it to a number again. If the parsing fails at any point, the
// number is just set to 0.
[CustomPropertyDrawer(typeof(HexIntAttribute))]
public class HexIntDrawer : PropertyDrawer
{
    public HexIntAttribute hexIntAttribute
    {
        get { return ((HexIntAttribute) attribute); }
    }

    public override void OnGUI(Rect position,
                               SerializedProperty property,
                               GUIContent label)
    {
        EditorGUI.BeginChangeCheck();
        string hexValue = EditorGUI.TextField(position, label,
             property.longValue.ToString(hexIntAttribute.FormatString));

        long value = 0;

        if (hexValue.StartsWith("0x"))
        {
            try
            {
                value = Convert.ToInt64(hexValue, 16);
            } catch (FormatException)
            {
                value = 0;
            }
        }
        else
        {
            bool parsed = long.TryParse(hexValue, System.Globalization.NumberStyles.HexNumber,
                                        null, out value);
            if (!parsed)
            {
                value = 0;
            }
        }

        if (EditorGUI.EndChangeCheck())
            property.longValue = value;
    }
}

// An example MonoBehaviour object showing how this Attribute could be used
public class InventoryItem : MonoBehaviour
{
    [HexInt(digits = 6)]
    public long id;
    
    public int value;
}