This is what I have been doing and it works OK. I'm planning a year long trip next year and feel a pack cover will be worth having. Not an issue in CO but in warm and humid climates I may not want to wear a rain coat but will still want to keep the pack contents dry. I've also done the garbage bag inside the pack but find it's sort of a pain to move everything and empty the pockets.
I understand. On such an important trip (for me) having the right gear really adds to the enjoyment.
With said you might consider making a custom pack cover that is really tailored to this trip.
hth