I did some tests with Dimension crabgrass control granules. I treated new grass seedling in flower pots at only only 7 days old, when they were 2 inches tall. I used perennial rye at near ideal seeding conditions--moist soil and temps in the eighties. There was no death of the seedlings, but there was some slight stunting.
Seedlings treated with Dimension at 23 days old and about 3 inches tall were not injured. Sept 2011.
I think it may be possible to treat new grass with Dimension if you can get it up to 3 inches tall before treatment. I hope to try this this year. Clearly there is a narrow window of time when the new grass is 3 inches and yet crabgrass has not yet germinated. I think the label says the new grass must be well-established. You must accept some risk.
If anybody tries this let us know what happens.