Would it be a good idea to rewrite the SVG renderer (which now is a compiled C program) as a PHP script that invokes the Linux command rsvg to render the SVG piece at the requested size and possibly the requested color (all specified in the PHP arguments), and relay the output to the client?
Converting an SVG to a PNG can be done in PHP without rsvg. The Diagram Designer does it by using Imagick to read the SVG file, convert it to PNG, and pass it to GD. If you just want a script to convert an SVG image to PNG, it could probably be done with just Imagick. The trick is to set the background color to a fully transparent color, which is any 8-digit hexadecimal number ending in 00.
Converting an SVG to a PNG can be done in PHP without rsvg. The Diagram Designer does it by using Imagick to read the SVG file, convert it to PNG, and pass it to GD. If you just want a script to convert an SVG image to PNG, it could probably be done with just Imagick. The trick is to set the background color to a fully transparent color, which is any 8-digit hexadecimal number ending in 00.