A simple MSBuild Task to modify the project file properties.

ProjectPropertyWriter.cs

using System.IO;
using System.Linq;
using Microsoft.Build.Construction;
using Microsoft.Build.Framework;
using Microsoft.Build.Utilities;

namespace EldSharp.Scripts.MsBuild
{
    public sealed class ProjectPropertyWriterTask : Task
    {
        [Required]
        public string ProjectFile { get; set; }

        [Required]
        public string Property { get; set; }

        [Required]
        public string Value { get; set; }

        public override bool Execute()
        {
            if (!File.Exists(ProjectFile))
            {
                Log.LogError("Project file {0} does not exist", ProjectFile);
                return false;
            }

            ProjectRootElement root = ProjectRootElement.Open(ProjectFile);
            if (root == null)
            {
                Log.LogError("Invalid project file: {0}", ProjectFile);
                return false;
            }

            var foundProperties = root.Properties.Where(p => p.Name == Property);

            foreach (ProjectPropertyElement property in foundProperties)
            {
                property.Value = Value;
            }

            root.Save();

            return true;
        }
    }
}