I see that it is possible to create a circular symbol (among other shapes) with SimpleMarkerSymbol. I also see that it is possible to create a text symbol with TextSymbol. Is it possible to combine these two to put a bit of text, centered, over the top of a circle element?
The easiest method is going to be to use two graphics layers with two different symbologies. (Or, if you have a large number of symbols and access to arcgis server, use a map service with the correct symbology underneath a graphics layer with blank symbols with no fill; that's what I do here: http://maps.stlouisco.com/police/beta)
It should be feasible to create a symbol class that does what you want. I want to do that for my application, so I will try to take some time later this week to figure that out.