Best thing to do is shift down sooner, while you are still pedaling easily.
Even though it generally isn't a great idea to shift under load I have seen people downshift under load without incident. Maybe the risk is better than a fall. Or better yet, if possible, stand on your pedals to get up a bit of speed, sit and shift down. But the real key is not letting things get to the point of no return and you do that by shifting down while it still is pretty easy to pedal.
I did a really steep hill yesterday and I went up in my lowest gear. I almost didn't make it up and had the same concern, that I would be going too slow to clip out. I managed to weave my way back and forth to get up the hill, essentially dealing with the hill like it had switchbacks. Of course, that only works if you have enough room to maneuver.
Trek Madone 4.7 WSD
Cannondale Quick4
1969 Schwinn Collegiate, original owner
Terry Classic
Richard Feynman: “The first principle is that you must not fool yourself and you are the easiest person to fool.”