Running any normal kind of bow across more than a half-dozen strings at the same time would only wiggle the strings at the edges, since the bow hairs have nothing to hold them down against the strings in the middle.

In order to shake all the strings on an autoharp it appears you would need something really stiff like a hacksaw rather than a bow (not necessarily a bad idea in this case?)

A mountain dulcimer takes on a whole different personality with a hammer rather than a plectrum, but bigger hammers won't make it much louder than plucking. The normal strings will take any reasonable amount of thumping with something like a small spoon, but with a typical setup the strings are too close together to pick out individual ones to hit, so most people only beat a melody on the top string "for the effect." Since you have to finger the strings, you can't easily use more than one spoon at a time, unless you've got a "courtin' dulcimer."

Another "effect" that can be used on some stringed instruments is to stoke the string lengthwise rather than cross wise. The sound wave velocity is about 6x as high for a tension wave as for a transverse wave for a typical string (which is why some guitar players squeak and chirp so often?). Look up the "barn door bass" for the traditional playground applications.

John