not sure what GPSAR is... but I have wondered how to do what youa re talking about also. I think any GPS will show you VMG if you pick a point and "go to" it. Even the one I had in 1996 did this.
My only idea is either to sail to weather and pick a point which can be seen or identified as the point when you are downwind. Mark that on your gps. go ddw and mark a spot there. Then you can have your GPS show your speed and VMG to either of those points.
I have seen some GPS that will actually pick a point in a direction 2 miles away or something if you give it a bearing from where you are now.
I am, in the end, too lazy to do this.