Anonymous participant The only 100% legal way is to obtain a work permit and register your work under it. Many agents may offer various solutions, but in most cases, they are not operating legally themselves.
No, you can't. You'll need a work permit and the appropriate visa. The DTV doesn't allow you to work in Thailand for or with Thai entities. Even working as a speaker for a U.S. company in Thailand still involves dealing with third-party entities. It falls into a gray area and may possibly be tolerated if it strictly targets employees of that company (your own colleagues) for informational purposes (not a paid seminar) and doesn't happen too frequently. Best is to check with the local immigration...
Being honest, I can't guarantee you this is happening, just some rumor, but I would keep a closer eye on it... We may see some more surprises. We already got plenty these days.