ASP.NET Web API 2.2 の IDirectRouteProvider を使用して、すべてのアクションに "{controller}/{action}" という UrlTemplate を付与した状態にしてみた。（が、全く実用的ではないし、converntion-base で一発でできる。）

ValuesController.cs

using System.Collections.Generic;
using System.Web.Http;

namespace WebApplication86.Controllers
{
	public class ValuesController : ApiController
	{
		// GET value/get
		public IEnumerable<string> Get()

		// GET values/get?id={id}
		public string Get(int id){ }

		// POST values/post
		public void Post([FromBody]string value){ }

		// PUT values/put?id={id}
		public void Put(int id, [FromBody]string value){ }

		// DELETE values/delete?id={id}
		public void Delete(int id){ }
	}
}

WebApiConfig.cs

using System;
using System.Collections.Generic;
using System.Web.Http;
using System.Web.Http.Controllers;
using System.Web.Http.Routing;

namespace WebApplication86
{
	public static class WebApiConfig
	{
		public static void Register(HttpConfiguration config)
		{
			config.MapHttpAttributeRoutes(new MyDirectRouteProvider());
		}
	}

	public class MyDirectRouteProvider : IDirectRouteProvider
	{
		public IReadOnlyList<RouteEntry> GetDirectRoutes(
			System.Web.Http.Controllers.HttpControllerDescriptor controllerDescriptor, 
			IReadOnlyList<System.Web.Http.Controllers.HttpActionDescriptor> actionDescriptors, 
			IInlineConstraintResolver constraintResolver)
		{
			// {controller}/{action} の direct route を作成する 
			var entries = new List<RouteEntry>();

			string controllerName = controllerDescriptor.ControllerName;

			foreach(HttpActionDescriptor action in actionDescriptors)
			{
				string actionName = action.ActionName;
				string template = String.Format("{0}/{1}", controllerName, actionName);

				var context = new DirectRouteFactoryContext(
					prefix: "", 
					actions: new HttpActionDescriptor[] { action }, 
					inlineConstraintResolver: constraintResolver,
					targetIsAction: true);

				IDirectRouteBuilder builder = context.CreateBuilder(template);
				var routeEntry = builder.Build();				
				entries.Add(routeEntry);
			}

			return entries;

		}
	}
}