Добавил сабж с некоторой степенью контроля над параметрами конструктора. Кому интересно — проглядите на предмет чего добавить/поправить.
Тесты:

using System;
using System.Collections.Generic;
using System.Linq;

using NUnit.Framework;

using static CodeJam.Reflection.ParamInfo;

namespace CodeJam.Reflection
{
    [TestFixture]
    public class CreateInstanceTests
    {
        [TestCase(typeof(ParamlessCtor), new string[0], ExpectedResult = "()")]
        [TestCase(typeof(ReqPrmsCtor), new [] {"a:aval", "b:bval"}, ExpectedResult = "(a:aval, b:bval)")]
        [TestCase(typeof(ReqPrmsCtor), new[] { "!a:aval", "b:bval" }, ExpectedResult = "(a:aval, b:bval)")]
        [TestCase(typeof(ReqPrmsCtor), new[] { "!a:aval", "!b:bval" }, ExpectedResult = "(a:aval, b:bval)")]
        [TestCase(typeof(ReqPrmsCtor), new[] { "!a:aval", "!b:bval", "c:cval" }, ExpectedResult = "(a:aval, b:bval)")]
        [TestCase(typeof(OptsPrmsCtor), new string[0], ExpectedResult = "(a:avaldef, b:bvaldef)")]
        [TestCase(typeof(OptsPrmsCtor), new [] {"a:aval"}, ExpectedResult = "(a:aval, b:bvaldef)")]
        [TestCase(typeof(OptsPrmsCtor), new[] { "a:aval", "b:bval" }, ExpectedResult = "(a:aval, b:bval)")]
        public string Test(Type type, string[] paramStrs)
        {
            var inst = type.CreateInstance(ParseParams(paramStrs));
            Assert.IsNotNull(inst);
            return inst.ToString();
        }

        private static ParamInfo[] ParseParams(IEnumerable<string> paramStrs) =>
            paramStrs
                .Select(
                    s =>
                    {
                        var parts = s.Split(':');
                        return
                            s.StartsWith("!")
                                ? Param(parts[0].Substring(1), parts[1], true)
                                : Param(parts[0], parts[1], false);
                    })
                .ToArray();

        [TestCase(typeof(ParamlessCtor), new[] { "!a:aval" })]
        [TestCase(typeof(ReqPrmsCtor), new string[0])]
        [TestCase(typeof(ReqPrmsCtor), new[] { "a:aval" })]
        [TestCase(typeof(ReqPrmsCtor), new[] { "!a:aval" })]
        [TestCase(typeof(ReqPrmsCtor), new[] { "!a:aval", "!c:cval" })]
        [TestCase(typeof(OptsPrmsCtor), new[] { "!c:cval" })]
        public void NoSuitableCtors(Type type, string[] paramStrs) =>
            Assert.Throws<ArgumentException>(() => type.CreateInstance(ParseParams(paramStrs)));

        private class ParamlessCtor
        {
            /// <summary>Initializes a new instance of the <see cref="T:System.Object" /> class.</summary>
            // ReSharper disable once EmptyConstructor
            public ParamlessCtor() { }

            #region Overrides of Object
            /// <summary>Returns a string that represents the current object.</summary>
            /// <returns>A string that represents the current object.</returns>
            public override string ToString() => "()";
            #endregion
        }

        private class ReqPrmsCtor
        {
            private readonly string _a;
            private readonly string _b;

            /// <summary>Initializes a new instance of the <see cref="T:System.Object" /> class.</summary>
            public ReqPrmsCtor(string a, string b)
            {
                _a = a;
                _b = b;
            }

            public override string ToString() => $"(a:{_a}, b:{_b})";
        }

        private class OptsPrmsCtor
        {
            private readonly string _a;
            private readonly string _b;

            /// <summary>Initializes a new instance of the <see cref="T:System.Object" /> class.</summary>
            public OptsPrmsCtor(string a = "avaldef", string b = "bvaldef")
            {
                _a = a;
                _b = b;
            }

            public override string ToString() => $"(a:{_a}, b:{_b})";
        }
    }
}