[Plugin: Geo Mashup] Marker labelling

You can assign your own markers using custom javascript: http://code.google.com/p/wordpress-geo-mashup/wiki/Documentation#Custom_JavaScript

Have a look at the custom.js used on this site for an example: http://www.natlands.org/geosearch/?zip=reading%2C+pa&search.x=0&search.y=0

It sounds to me like you probably don’t need to use a filter, but you do need some custom javascript to set an object marker as that example does. The code will be different – more like the example I linked to perhaps. The key is that when you give an object a custom icon in your custom javascript, that icon will be used in the list also.