Pit is right, this isn't somethign we expose on teh API at the moment. However, the ShinobiGrid subclasses UIScrollView, so there is no need to 'hunt' for the UIScrollView ... you can scroll the grid directly as follows:
[grid setContentOffset:CGPointMake(0, 200) animated:YES];
You'll have to work out a suitable offset based on the height of your rows. Hope that helps.
(And yes, we do have some big plans for our nest grids release!)