Check out
Mountain Tamer Triple adapter - LOWER GEARS for bikes of all kinds
It lets you use rear cogs as small as 17 as chain rings.
I have to say that I have a hard time imagining anyone needing rings that small though. Back in the days that I packed pretty heavy, I did the Trans America with a 26/36/46 and an 11-32 and didn't wish for lower gearing in the Cascades or Rockies. There were a few places in the Appalachians where a 24T ring might have been nice, but it was OK without it.
Since I pack lighter now I don't even feel the need to go as low as I did for the TA, in fact on the Southern Tier I used a 39/26 with a 12-28 and found it perfectly adequate . Despite having camping and cooking gear, I was packed VERY light though. I was 61 year old for that ride so it isn't like I am a fit 18 year old.
The climbs were pretty crazy in the Sierras on the Sierra Cascades route so I might want lower gearing there, but still can't imagine wanting lower than what you have on your bike now.
Worst case you can walk a bit. That can actually be a nice change of pace. Since most places where the climbs are crazy steep they are also short. If I needed less than 22T ring with a 34 cog, I would just as soon walk. I don't usually need to walk, but sometimes like to even where it isn't necessary just for the change of pace.