Depends on how long it's 14% for! In the Peaks I generally cope with 39/28, until it gets nearer 25-30% and I'm a fat git, so you might find you're worrying more than necessary
I was just checking out that cool graphic again, and really there's only the section 0.15-0.4 that's going to trouble you. That's quarter of a mile total, less than 400 metres out of the whole day's riding.
I think you're gonna be just fine - save your cash for a sugary snack a couple of miles before you hit it. Or a beer, that often works well for short bursts...
Just going to have to decide which loses you more man points, using an MTB cassette/chainring, or walking for a couple of hundred metres.
(after manfully attempting to ride of course!)