Got at least one data fetching method working; turns out, we can't use a patched LogicStack to get the data

Vendor/EmbedIO-3.5.2/RequestDeserializer.cs

@@ -0,0 +1,64 @@
+using System;
+using System.Threading.Tasks;
+using Swan.Formatters;
+using Swan.Logging;
+
+namespace EmbedIO
+{
+    /// <summary>
+    /// Provides standard request deserialization callbacks.
+    /// </summary>
+    public static class RequestDeserializer
+    {
+        /// <summary>
+        /// <para>The default request deserializer used by EmbedIO.</para>
+        /// <para>Equivalent to <see cref="Json{TData}"/>.</para>
+        /// </summary>
+        /// <typeparam name="TData">The expected type of the deserialized data.</typeparam>
+        /// <param name="context">The <see cref="IHttpContext"/> whose request body is to be deserialized.</param>
+        /// <returns>A <see cref="Task{TResult}">Task</see>, representing the ongoing operation,
+        /// whose result will be the deserialized data.</returns>
+        public static Task<TData> Default<TData>(IHttpContext context) => Json<TData>(context);
+
+        /// <summary>
+        /// Asynchronously deserializes a request body in JSON format.
+        /// </summary>
+        /// <typeparam name="TData">The expected type of the deserialized data.</typeparam>
+        /// <param name="context">The <see cref="IHttpContext"/> whose request body is to be deserialized.</param>
+        /// <returns>A <see cref="Task{TResult}">Task</see>, representing the ongoing operation,
+        /// whose result will be the deserialized data.</returns>
+        public static Task<TData> Json<TData>(IHttpContext context) => JsonInternal<TData>(context, default);
+
+        /// <summary>
+        /// Returns a <see cref="RequestDeserializerCallback{TData}">RequestDeserializerCallback</see>
+        /// that will deserialize an HTTP request body in JSON format, using the specified property name casing.
+        /// </summary>
+        /// <typeparam name="TData">The expected type of the deserialized data.</typeparam>
+        /// <param name="jsonSerializerCase">The <see cref="JsonSerializerCase"/> to use.</param>
+        /// <returns>A <see cref="RequestDeserializerCallback{TData}"/> that can be used to deserialize
+        /// a JSON request body.</returns>
+        public static RequestDeserializerCallback<TData> Json<TData>(JsonSerializerCase jsonSerializerCase)
+            => context => JsonInternal<TData>(context, jsonSerializerCase);
+
+        private static async Task<TData> JsonInternal<TData>(IHttpContext context, JsonSerializerCase jsonSerializerCase)
+        {
+            string body;
+            using (var reader = context.OpenRequestText())
+            {
+                body = await reader.ReadToEndAsync().ConfigureAwait(false);
+            }
+
+            try
+            {
+                return Swan.Formatters.Json.Deserialize<TData>(body, jsonSerializerCase);
+            }
+            catch (FormatException)
+            {
+                $"[{context.Id}] Cannot convert JSON request body to {typeof(TData).Name}, sending 400 Bad Request..."
+                    .Warn($"{nameof(RequestDeserializer)}.{nameof(Json)}");
+
+                throw HttpException.BadRequest("Incorrect request data format.");
+            }
+        }
+    }
+}