Have the bag mailed to you via general delivery. Return when no longer needed. The post office will hold the pkg for maybe 10 days before returning to sender. When I've done this, I put a note on the pkg "On bicycle tour. Please hold for pickup" to encourage hold time leniency. Never had a problem with this method.

I always pack a pair of wind pants and a light pull over to supplement the flannal sheet bag I carry in the summer in case the temps drop a bit lower than expected. Not unusual to sleep on top of the sheet on especially warm nights. Had a bunch of those last summer in the north central US.