It is so easy, write the code that you want in safe mode C++/CLI, get the code template, then implement the helper classes to generate the same MSIL on the fly.

As for stack allocation, apparently you missed structs.

Here is your string allocated on the stack.

    unsafe struct cppstring
    {
        const int BuffSize = 1024;
        fixed char data[BuffSize + 1];
        int current;
        
        public cppstring(System.ReadOnlySpan<char> buffer)
        {
            for (int i = 0; i < System.Math.Min(BuffSize, buffer.Length); i++)
                data[i] = buffer[i];
            current = 0;
        }
    }

    public class StackDemo {
        public void Myfunc() {
            var str = new cppstring ("Hello from stack");
        }
    }

Providing std::string like operations is left as exercise for the reader.