Is this possible with Bryce? Are would I have to attempt to paint grass in the post-work? I haven't found a good method of making realstic grass..

Grass is a hard one, Here's a place which has some grass models http://www.3dplants.com/ They work good for patches, but you can't really fill up the whole scene with them. Another option is to use a flatten cube or even a 2D plane with a decent tex & trans map for distant grass ( doesn't work to good close up ) Infinite planes don't work to good for this, because the tex map gets to stretched out. Then there's always the mask render and post work.

you can make a new terrain--go into the terrain editor, hit new and then hit spikes a time or two. Instant patch of grass...

forgot to mention you might want to make the grid size about 512-ultra fine for close grass.

Yes the Spikes in the old terrain editor works pretty good. For more detailed grass 3dplants.com is the place for free grass meshes.