The best solution to this appears to dimension the plan in Sketchup. Before flipping the plan (Scaling -1), put all of the dimensions into a group. Scale both the plan and dimension groups together and the dimensions will appear correctly in the reversed plan.
With this approach, you can get a reverse reading plan in just a few key strokes. Warning - exploding the dimension group will result in unpredictable results with some dimensions reverting to their former location.
You should also try putting the dimensions and plan into the same group before scaling. This appears to work as well but the dimensions are a bit odd if you attempt to make adjustments to the plan.