Just noticed this new support for customizing map's visible scales: type in the scale you want such as 200 and click the plus sign then you can zoom in beyond the base map.
You may want to consider using the same type of steps in the scale, like:
564
282
141
The scale value reduces to half of the previous scale value in each step. This makes visualization smoother...
We use it to show network data on the our clients aerial photographs, which have much higher resolution than the existing basemaps.