Yes, it is possible to average a waypoint on the Quest! Another omission in the manual and somewhat hard to discover though. The catch is, it only works for new created waypoint, not for existing waypoints. Here is how to do it:
You start by creating a waypoint, by holding for a moment. It now searches for nearby roads and displays your current location. You then press another time to Save this location.
This should all look familiar until now, since this is how you always save a waypoint. =8-) But, now press and see what happens!
Yes! There is a menu entry to Average Location. Pressing displayes the actual averaging screen.
Here you will see the (changing) location, the number of samples the Quest took until now and the estimated accuracy of this averaging. If you are satisfied with the estimated accuracy, press again to save the location.
Remember: averaging needs to be done with a good view of the sky (e.g. the best accuracy you can get in the first place) and for several minutes (if not tens of minutes). And, since the Quest stores its location with a certain accuracy only, you won;t get centimeter accuracy... expect to get down to less than 2 meters at best. If WAAS/EGNOS is active you can also get that kind of accuracy without averaging.