Can you provide a few examples of good integrations?
English-by-phone.ru is probably the best integration that uses all the potential of the Zamenhof system.
Inglestogo is a really well made website. They integrated the free evaluation registration page by using html code. They are selling exclusively open courses. Note the popup window from the homepage; it did a great job in converting the visitors into students.
English-by-Phone.ru is one of the most effective websites from the network; they hired a famous Russian personality, Vasily Strelnikov to promote the service. This has an incredible impact on the visitors.