A mortiser chain has some similarities to a saw chain, but it's also very different. Since the Makita cuts with the grain rather than across it like a chain saw, the teeth are filed straight across, not at an angle.
I've got a Ryobi mortiser, similar to the Makita, and found I can file each row of teeth with a chain saw file, straight across. I wouldn't recommend touching the face of the teeth with a straight file, just the curved inner portion, with a file that matches the radius as closely as possible. Be very careful not to change the shape of the teeth, file an even amount from all the teeth, and keep the file as straight as you can.
You should consider this a touch-up, to be done occasionally between machine sharpenings, which will remove the irregularities from hand filing. As with a chain saw, I'd touch-up the chain two or three times, then send it in to get it evened up by machine.
Local sharpening shops probably have never had anything similar in their shops; if you know someone reputable they may be persuaded to develop the specialized set-up needed, but no one else will be sharing the set-up costs. I'm not familiar with Miller's, but would think going to someone with experience would be a good idea.
In order not to remove too much material, I always told the shop it was OK if they stopped before all the teeth cleaned up, if there were some excessively worn, or filed too far by hand. You probably won't notice the difference in the cutting action if it's just a few teeth. Those teeth won't wear if they're not cutting, and will catch up on subsequent sharpenings.