The smaller problem is that ImageMagick seems to rasterize the SVG file as soon as the image gets loaded. I think there's a way around that though. If worse comes to worse, I can always alter the SVG XML in memory before the load. Luckily SVG declares some simple transformations that can be used to wrap the nodes.
The bigger problem is that ImageMagick doesn't seem capable of rendering what I thought was a fairly simple SVG file properly. Inkscape and Adobe's SVG viewer both seem to produce what I expected, so I'm guessing this is a deficiency in the ImageMagick SVG rendering.
Oh well, my release date isn't for another seven years; I can probably work out a proof on concept using a reduced set of SVG functionality and hope a better rasterizer comes along later.