I can share with you my experiences. Redwoods produce a dense fibrous mat of roots that spread out far beyond the drip-line. The roots are so dense that almost nothing will grow that requires ground water for sustenance. Because redwoods are so very thirsty, the roots will actually steal water from the plants that are within their reach. So what ever you plant must get all the water they need from frequent surface waterings. The plants that do grow well within the root zone of redwoods are those that have an ability to take in water through their leaves and thus get their water needs from the air on foggy misty summer days. Inland, you pretty much need to re-create those foggy misty conditions several times each week, and if you do, will have success with numerous natives, such as ferns, rhododendrons, sorrel, some iris and other tubers, and weedy annuals including clumping grasses.
But I have a suspicion many shade/acid lovers will do well if you can keep the water needs met.
Take a look at this informative article by a Master Gardner in the Marin Independent Journal.
http://www.marinij.com/lifestyles/ci_14523545
