You might be right to be worried about maxillo-facial surgery but not because you don't know any dentistry...
nexttime is correct that you will spend most of your time doing paperwork. The reason to fear max fax is that it is often poorly supported as the SpRs/consultants are in theatre and the other SHOs might not be medically trained. If you are in a large hospital, the max fax surgeons might do some pretty heroic operations - large neck dissections, tracheostomies, free flaps from the lower limb, etc. These patients can be complicated and get very sick post-operatively. You just need to do the basic things (A-E) and be assertive about calling for help (SpR/consultant/medical SpR/ITU/critical care outreach/whoever!).
I suspect you'll end up being "on call" (i.e. seeing emergencies) for max fax and, in most hospitals, the max fax, plastics, and ENT SHOs share the same rota. No-one really expects you to make decisions about patients for any of the specialties - you just see the patient and call the appropriate SpR to ask what to do next. By the end of the job you'll be reasonably confident about the most common presentations. Max fax "emergencies" are all dental abscesses (painkillers and advise to see a dentist if systemically well; IV antibiotics if unwell), bleeding after dental extractions (apply pressure is all you can really do although pressure with adrenaline-soaked gauze helps), and fractured mandibles (admit, NBM, IV antibiotics [treated as an open fracture as in continuity with the mouth]). ENT is all epistaxis (reverse warfarin and pack nose). Plastics can be complicated because it covers the whole spectrum of wounds and hand injuries. The good news is that most injuries can be wrapped up until the next day when someone senior is around to make decisions.
You will be fine, though - honest. I don't think a max fax job (even if it includes ENT and plastics) should be as terrifying as suddenly being the general medical or general surgical SHO.*