No just in degrees. I tend to only support map services in degrees and polar only. I am fairly adverse to Web Mercator. Hopefully that trend will die as Google moves to a globe viewer. I always understood Web Mercator was originally used such that directions were more "cardinal" (north is up, east to the right) for driving applications. 

By using degrees, we can cheat and allow applications which don't understand planets to at least draw the layers. That doesn't mean it is good and there is always the potential for measures to be reported incorrectly. But it seems more thick clients (QGIS, ArcMap, etc.) will do the right thing and project correctly from degrees to a (i.e.) a Mars defined map projection within the application. For the Esri served layers (1TB CTX, 19TB Hirise, etc.), they are tagged as on Mars, but they use Esri codes. These Esri codes are in GDAL (including most planets), but that doesn't mean all viewers are going to use them. Thus for web clients, we have needed to modify the code in specific places (like the measure tool) to understand the body we are currently one (e.g. open layers, leaflet).

The latest on IAU projection codes is that they will be finally available in the Proj (v6) library within a few months (I hope). That won't solve a ton of issues but it should allow for programs which use GDAL to more easily use them, then we can work on the web clients...