I did an emergency dismount on this 25% ramp. I didn't even attempt to get started again - I walked up to the bend and started off on the easier slopes higher up!
If you look closely at my bike in the photo in that post, you will see that I had Look pedals at the time. The experience of my feet slithering about when walking up to the top of that ramp was enough to convince me to put double-sided mountain bike type SPDs on.
With SPDs I would have felt okay starting across the width of the road with one foot clipped in. I'd be confident of clipping the other foot in, or at least it being safe to have to put a foot down again.
I had almost faceplanted when dismounting wearing Look-cleated shoes. My foot slipped away when I put it down!