After some reading of the SVG reference document on W3C, I realised that
the <image> element allows the inclusion of entire files, be they in
JPG, PNG or SVG (!) format.
Yes the solution is just to link the SVG file as an
image to your SVG file. Link the background to your slides.
As a feature request, it would be great if the import function would
allow me to choose, how I import an SVG file. Either have it parsed (the
current implementation in inkscape) or leave it as an <image> tag as is
done for JPG and PNG imports.
Have you filled an issue to request that feature yet ?