At least that can be done in SVG with a radial gradient (I don't know whether Inkscape exposes it). But for anything more complex than an ellipse, you're out of

At least that can be done in SVG with a radial gradient (I don't know whether Inkscape exposes it). But for anything more complex than an ellipse, you're out of luck, short of something like a filter effect, which will be very heavyweight and also inconsistent across browsers.