If you had your own features marked with a tag like “mark_lesters_own_feature=yes” you could select and show it from my server as easily as you can now show, let’s say “highway=bus_stop”, and without you or me doing anything for the system.

http://hip.latuviitta.org/cgi-bin/ms_ows?REQUEST=GetMap&SERVICE=WMS&VERSION=1.1.1&WIDTH=563&HEIGHT=437&LAYERS=sea,borders,osm_pisteet&TRANSPARENT=TRUE&FORMAT=image/png&BBOX=-369151.98300283286,6597900.0,1511076.628895184,8057331.444759207&SRS=EPSG:3067&STYLES=&sql=%28tags%20@%3E%20%27highway=%3Ebus_stop%27%29

I am sure that Mapnik could be used as an engine to this kind of flexible services. Unfortunately rendering transparent tiles seems to be the only solution that people consider.