Suppose a GA pilot tows a banner that promotes the cause of some
non-profit org. Would the costs of that flight be a tax deductible
charitable donation?

You do know that it takes an FAA waiver to tow banners?
In general the FAA doesn't give a rats ass what you deduct on your
taxes. There were some rumblings to the contrary from time to time
but they usually get promptly squashed. You need to follow the IRS
rules for non-monetary donations.

Another option: contractors working temporary duty far from home and
not getting per diem can deduct travel costs for flying to their
residence to the extent of the per diem rate in their temporary duty
location.

Certainly. And it actually works well if you are an independant
contractor and can take those off the top on your sched c. keep
good contemporaneous records.