Some moss and a small randomly-placed anubias nana var petite would look nice on the rock and would 'soften' it's appearence. Large-leaved plants in the foreground can do wonders for adding depth. (google "forced perspective")
How much light do you have on that tank? If it's a good bit, you could try some dwarf baby tears on the substrate and allow it to cover the foreground.
Hairgrass is popular in Iwagumi-style tanks, but Echinodorus var Vesuvius would look nice in my opinion-
Just pull out the large ones when it propogates and you'll have a number of small plants that look almost like a cross between twisted val and dwarf hairgrass. (You could also grow the vesuvius in another tank and transplant the small runners that form when they are a couple inches tall)
The iwagumi tank doesn't lend itself too much to soil- Personally, I'd do lots of waterchanges for the first few months to get rid of surplus nutrients.
For color, you could use a few stems of Rotala Rotundifolia or cabomba furcata. Ooh, I just saw the floating plants (azolla? duckweed?). They'll be perfect to absorb nutrients for now, along with the stem plants I mentioned. In a month or so, you can fish the floating plants out and try baby tears.