Evil Fody weaver for force writing readonly fields with test program to be weaved by it.

ModuleWeaver.cs

using Fody;
using Mono.Cecil;
using Mono.Cecil.Cil;
using System.Collections.Generic;
using System.Linq;

namespace Evil.Fody
{
    public class ModuleWeaver : BaseModuleWeaver
    {
        public override void Execute()
        {
            TypeDefinition programType = ModuleDefinition.Types.First(t => t.Name == "Program");

            FieldDefinition lolField = programType.Fields.First(f => f.Name == "<Lol>k__BackingField");
            FieldDefinition lelField = programType.Fields.First(f => f.Name == "Lel");

            MethodDefinition evilConstructor = programType.Methods.First(m => m.Name == ".ctor" && m.Parameters.Count == 0);

            MethodDefinition evilMethod = programType.Methods.First(m => m.Name == "Evil");
            evilMethod.Body.Instructions.Clear();
            ILProcessor processor = evilMethod.Body.GetILProcessor();
            processor.Emit(OpCodes.Nop);

#if true
            // This produces verification error 0x80131884
            // [IL]: Error: [... : ForceWriteReadonly.Program::Evil][offset 0x0000000A]
            // Cannot change initonly field outside its .ctor.(Error: 0x80131884)
            processor.Emit(OpCodes.Ldarg_0);
            processor.Emit(OpCodes.Ldarg_0);
            processor.Emit(OpCodes.Ldfld, lelField);
            processor.Emit(OpCodes.Ldc_I4_1);
            processor.Emit(OpCodes.Add);
            processor.Emit(OpCodes.Stfld, lelField);

            processor.Emit(OpCodes.Ldarg_0);
            processor.Emit(OpCodes.Ldarg_0);
            processor.Emit(OpCodes.Ldfld, lolField);
            processor.Emit(OpCodes.Ldc_I4_1);
            processor.Emit(OpCodes.Add);
            processor.Emit(OpCodes.Stfld, lolField);
#else
            // This produces verification error 0x801318BF
            // [IL]: Error: [... : ForceWriteReadonly.Program::Evil][offset 0x00000002][found ref ('this' ptr) 'ForceWriteReadonly.Program']
            // Call to .ctor only allowed to initialize this pointer from within a .ctor. Try newobj.(Error: 0x801318BF)
            processor.Emit(OpCodes.Ldarg_0);
            processor.Emit(OpCodes.Call, evilConstructor);
#endif

            processor.Emit(OpCodes.Ret);
        }

        public override IEnumerable<string> GetAssembliesForScanning()
        {
            yield break;
        }
    }
}

Program.cs

using System;

namespace ForceWriteReadonly
{
    class Program
    {
        public int Lol { get; }
        public readonly int Lel;

        public Program(int x, int y)
        {
            Lol = x;
            Lel = y;
        }

        // Evil constructor for alternate method
        private Program()
        {
            Lol++;
            Lel++;
        }

        public void Evil()
        {
            //Lol++;
            //Lel++;
        }

        public override string ToString()
            => $"{Lol}, {Lel}";

        static void Main(string[] args)
        {
            Program p = new Program(100, 200);
            Console.WriteLine(p);
            p.Evil();
            Console.WriteLine(p);
            p.Evil();
            Console.WriteLine(p);
            p.Evil();
            Console.WriteLine(p);
            Console.WriteLine("Done.");
            Console.ReadLine();
        }
    }
}

Here's the output of the weaved program:

100, 200
101, 201
102, 202
103, 203
Done.

Also maybe worth pointing out that I've tested this in .NET Framework as well as .NET Core using Fody 3.0.3.