I think if you go slow enough, 30-45mph, and it is warm enough out, 70F or so, this would be no problem. Any faster or colder out would probably be pushing it. It is a shame the Idaho Springs charger isn't more reliable. If you read my post in the Denver to Grand Junction thread you'll see I made it to Idaho springs with 49% remaining after driving 44 miles to get there. And from there I used 49% to get to Eisenhower tunnel which is a similar distance and elevation gain as Berthoud. I was going pretty slow on the way to Idaho springs though since there was so much traffic we were doing probably 30 on average and a bit of stop and go. After Idaho springs I did do 50-55 most of the way though. I drive a '15 with minimal to no battery degradation and it was 80+ degrees out back when I did it.
2015 Leaf S with QC, purchased July '14
35000mi as of August-17, 92% SOH