Hey, Karsten.
Using pure Mapserver, you may be able to fudge something using a label
and setting the label's position. If the label is centered, and the icon
is big enough, and you tweak the label's font and size, it may look good
enough for your needs.
Other than that, I think you're out of luck. Mapserver uses predefined
symbols and predefined classes. Using Mapscript, you get a lot of wiggle
room as far as, for example, dynamically defining coordinates to define
a new symbol on the fly.
--
Gregor Mosheh / Greg Allensworth
System Administrator, HostGIS cartographic development & hosting services
http://www.HostGIS.com/
"Remember that no one cares if you can back up,
only if you can restore." - AMANDA
