In expression maps, how does one set the default articulation?

More specifically for a violin VST; just plain arco for use when nothing else is specified in the score. (Assuming that the VST has a “plain arco”, of course.)

the default articulation is called “natural” (or “ord”). Simply create a switch in your Expression Map which uses that technique and it is automatically played whenever you want to do a reset. Most people would chose a generic sustain technique but in theory you can pick any articulation from your library you want.