Intra....Oral you say?
I have training in both intra- and extra-oral work (providing both for TMJ issues) but have found that since I am able to get very good results with my extra-oral work, I only rarely use intra-oral work for the TMJ issues anymore.
Personally, I wouldn't advertise this work. However, like I said, that is a personal preference, as I don't like to do it unless I find it absolutely necessary. I wouldn't want people calling me and asking for this work. Instead, I would advertise my TMJ work and then explain the intra-oral work to the client if I thought it was necessary for their sessions.
As a side note, I have had work done inside my nostrils by a Rolfer, and found it to provide a depth into my emotional state which I would not have expected. Possibly (and I am just making the stretch between the two items), you will find that the intra-oral work will provide you an adjunct to any clients you have with emotional stressors, and could offer it those types of clients too. Again, not advertising the intra-oral work as such.
If you are set on actually advertising the work, I would consider putting a special page on your website which shows a picture of the work, so that people are not confused (because I do believe that you could end up with strange calls otherwise, that you may not have time to field) and state something like: "Massage therapy techniques inside the mouth can be beneficial for TMJ dysfunction and headaches.
I have completed the specific 16-hour educational requirement which must be met by LMP's in Washington State and received my endorsement to practice this work.
If you would like to schedule an appointment for this work, I invite you to contact me with any specific questions."
And/or, state what other information you would like your clients to know (about the work) on the page.