I top off my tanks all the time... I have my reasons.
If you all must know... And from some of the responses so far, I guess you do...
I always boondock, often in remote locations for long durations. Going to get propane could mean a round trip of 200 miles, and a days hunt wasted.
So I want everything as full as I can get it.
To the OP. I can think of no reason why a gravity fill setup would not work just fine. And the Overfill protection device on the tank being filled should prevent it from filling past 80% just as it does when being filled with a pump.