You can create an App Engine POST task with a JSON body as well as a form-encoded one. You just need to set the Payload and Content-Type.

taskqueue_json_task.go

package main

import (
	"encoding/json"
	"net/http"

	"google.golang.org/appengine/taskqueue"
)

// NewJSONPostTask creates a Task that will POST to a path with the given body
// encoded as JSON.
func NewJSONPostTask(path string, body interface{}) (*taskqueue.Task, error) {
	h := make(http.Header)
	h.Set("Content-Type", "application/json")

	b, err := json.Marshal(body)
	if err != nil {
		return nil, err
	}

	return &taskqueue.Task{
		Path:    path,
		Payload: b,
		Header:  h,
		Method:  "POST",
	}, nil
}