I always think it is worth watching and listening to the builder play their own instruments if at all possible, as it can open your eyes and ears to why they build and tune the way they do. Three of the 5 videos here show JIm playing:http://mastertonedulcimers.webs.com/apps/videos/
Watching Jim, he reminds me a lot of ID Stamper in both his dulcimer design and playing style, particularly his right hand work with a flat pick. Those wide, shallow, high tuned dulcimers certainly kick out some sound! ID Stamper also would sometimes put a B note into his drones in G tunings, though not always. From the videos that Jim has chosen to present it looks like he plays mostly noter drone in the major key from his prefered tuning. And this is quite classic for traditional playing - I have found other examples (like the Graves family) who have used only one tuning and played only major key tunes for over a 100 years. And the key of G seems to be quite prevalent for this practice (Stamper's short scale, Graves walking cane, Melton Galax style). From looking at the videos, when Jim chords he simply uses the 4th chord at the 3rd fret (3,0,3) and 5th chord at the 4th fret (4,0,4) keeping the middle string open as a drone. The high B string drone alongside the bass G string will be in harmony when fretted at the 3rd and 4th fret for the 4th and 5th chords. If he plays only major folk tunes then Jim is unlikely to need any more chords than these two. It is a very simply yet very effective way to play the instrument.
Of course, any owner of a Mastertone could fit other string gauges and use other tunings and other playing styles. But if I owned one then I think I would keep it for playing tunes using the same styles as Jim and ID Stamper.