We are working on getting more greenery around our rocks here too, so I know what you're dealing with. I don't know if you are a fan of sedum at all, but any of them would do very well in the situation you describe, especially if there is very little soil.

The taller types, such as 'Autumn Joy' of 'Matrona' may require a bit more soil depth but have consistently amazed us with their capacity to flourish; the more groundhugging varieties such as 'Dragon's Blood' or 'Voodoo' or hens & chicks would be happy with an amazingly small amount of soil. There is an incredible variety of sedum cultivars and they are very tough plants which also add winter interest.

Depending on the look you like, the perennial grasses such as 'Karl Foerster' or any of the Switch Grasses (panicum) or Miscanthus can soften the outline of the stone and make it look more natural.
For more bloom, daylilies are quite tough and come in a wide range of colors; dwarf Russian Sage w/ it's silver foliage and blue spikes, any of the yarrow plants also. I have interplanted echinaceas, such as 'Kim's Mophead' and 'Kim's Kneehigh' in amongst our rocks with good success too.
There are many alpine plants that would work also, but I am not very familiar with them and need to do more homework. Some of the smaller shrubs might work well too. I've been visiting the rock garden/alpine plants forums and have been amazed at the variety of plant material. Good luck with your project! ;-) Karen