Overcoming Rackspace Cloud Files 10,000 object limit
Found the spot to hack this. It was not within the AssetIndexTool as originally theorized, but in the  In RackspaceAssetSourceType in the _getFileList function you can append a marker marker_end and/or limit to the file list.

The marker should just be a URI without the extension. As an example, we change the parameters on $targetUri to look like so:

&amp;format=json&amp;marker=products/BC-174&amp;limit=250';

This will index the items form that marker and effectively ignore all the other assets you have already indexed. Craft will prompt you to delete the items you already have, as they are not found because you have skipped them by hacking the $fileList.

I've found you can safely hit Cancel and your items from the marker range will be imported, and your other indexes will not be affected.

This article is very helpful in finding out what you can do with the marker params.