Nick,
Had a look at your site, not sure what projection/ellipsoid multimap are using.
WGS84 is an ellipsoid, not a projection. I believe all you would need to do is transform the co-ordinates from WGS84 ellipsoid to whatever ellipsoid multimap is using, then just plot the coordinates. (it might be something simple like the Clarke 1866)
A useful set of transformations and projections can be found in the proj4 package
http://www.remotesensing.org/proj/
This is similar to what they did for the great circle mapper @
http://gc.kls2.com