Similar low woodland or forest carpeters. Maybe try Viola sempervirens, I've seen that in large bare areas under Thuja plicata - and it's a cute plant.

The shrubs won't like much digging into their roots, nor will they want to be buried with a new layer of soil.

I have ferns, hostas, ajuga, and dicentras combined with my rhodies.

I plant from 4" pots to reduce root disturbance.

Hardy Cyclamen looks good for me.

Vancouveria

We grow a large Turkish Squash,called Adapazari Squash.You can read about it on the GardenWeb/Heirloom Forum. The leaves are Large Dark and Beautiful. It gets 20-30 ft in length and we weaveg it in and around Rhodies and other Plants. At the end of the year you will have one large Squash per plant. My biggest last year was 54 lbs. If anyone interested i have plenty of seeds I could send for free and no Postage necessary.
Jim

Wintergreen! (Gaultheria procumbens) Great berries.

If you'd like some Lily of the Valley you are welcome to visit me and dig some up. FREE. ;) ;) ;)

Abkhazi garden in Victoria has half a garden under mature rhodos. Arisaema, geranium, podophyllum, cyclamen, viola, erythronium, uvularia, polygonatum, dicentra... the list is endless. Forest understory perennials is what you want.

Cornus canadensis has crept under my large old rhodie and is very sweet.

I also have English bluebells under another, planted by the previous owner. They aren't my favorite because they self-seed and even small ones are almost impossible to pull, but they look great when they bloom at the same time as the pink rhodie. I am trying to remember to rip them out, foliage and all, before they go to seed and hope to control the spreading somewhat that way. Other than that, they are absolutely zero care.

If you go up to the Seattle area, stop on your way at the Rhododendron Species Botanical Garden in Federal Way. They've got 22 acres of rhodies underplanted with more things than I can enumerate.
I recently acquired some Beesia, an evergreen groundcover with little white flowers. It looks a lot like Asarum. I'm also a huge fan of hardy cyclamen.