Hi, did you ever get this question answered? I usually download .tiff files from the Library of Congress Sanborn collection, georeference them, save the rasters into a mosaic database, create a tile cache, export a package and share to ArcGIS Online. There are lots of details in these steps of course. If you are interested in working on this more, let me know. The URL where I post my Sanborn overlays is below.
Also, I have gotten lots of good help when I posted questions about the Sanborns in the "Imagery" community.
The Sanborn Overlays – Introduction | Olympia Historical Society and Bigelow House Museum
