Just created an app with a collection of scanned maps of New Haven. Cached it using the GE/ArcGIS Online scheme, but added one more level of zoom at 1:564.248588 to allow users to zoom to a legible level. I've got the layer added in Flex Viewer, but I find that I am locked at the 1:1128 scale and can zoom no further. I can't see any way to enable the additional zoom level in the xml config files for the viewer, or the nav widgets. Is there really no way to add a zoom level without recompiling the viewer code? You guys couldn't have given us ONE MORE zoom level, huh?
If your new map service is the same spatial reference as the the other base maps than all you have to do is ensure that your map service with the extra LOD is the first added to the map and its LODS will be used for all other map services that are added to the map. So try adding yours first to the basemaps list in your config.xml
