Implementing a a completely general mechanism for calculating the area of a closed SVG path would be a serious pain. I might make an attempt at something that could do it for a limited class of paths (strict polygons)

If you want to try something a bit more exotic, you could try developing your raw map data in a GIS like QuantumGIS, then export to SVG and 'pretty it up' in Inkscape. Then you would have access to tools like area calculation, overlay operations, projection transformation, etc. It does mean learning to use another, fairly complex bit of software though.