When you say you are a skilled mechanic, do you mean amateur or professional? If you are professional then you are working for "a firm" and therefore you can stamp your own service book with a genuine stamp.

In that case Yamaha would have no argument and your warranty status is safe.

If you are an amateur mechanic, then you've got no access to a stamp, you are compelled to have an authorized Yamaha or similar dealer to do the service for you during the first 2 years of your bike life.