Is "fly parallel to V157 offset 2 miles to the left" something
that I might ever expect to get in an IFR clearance if I file /G ?

One of the concerns with GPS is that the great accuracy can sometimes increase
the probability of a midair collision compared to conventional nav. Consider
two aircraft traveling on the same route in opposite directions at different
altitudes, in non-radar airspace. If, due to a pilot or controller error,
both planes end up at the same altitude, the probability that they would hit
was quite small with VOR, but much higher with GPS. Some airline pilots have
already started flying lateral offsets on their own, and ICAO is studying the
issue and trying to come up with standards. One of the proposed criteria is
that the procedure be transparent to the controllers. So it's unlikely that
ATC will ever assign a clearance to fly an offset, but at some point pilots
might be permitted or required to fly an offset on some routes.