I hear you, sounds like you are looking for a real stump-puller. I've had good success with a build I did just for a hilly tour with a 48/34 compact and a nice big 11-34 cassette. The 1:1 low gear was plenty for anything I threw at it. But I wasn't pulling a Burley, so I can't say what would work for you. Basically, if you are willing to sacrifice a skosh of top end (which this bike's purpose says you are) there's really no (practical) limit to how low you could go with a double.
Can't wait to see pics when it's finished. Sounds like a fun, capable bike.