Hello Bruno,
Having been in China for quite a while I understand the situation you're in using low bandwidth.
It would be way cool to have external style sheet support in Inkscape and something that has been on my wish list for quite a while, so please continue. Even if it's ugly coding and creates a performance hit when loading Inkscape, I think it would be worthwhile to have it. Maybe using a switch in the prefs to turn it on or off, for performance improvement.
Added to that is another question.
How feasible would it be to use the external style sheet support code you're currently making to do the same for external <USE xlink:href="file.svg#objectID"/> ? I have the idea that it would be pretty similar in scope, reading data from file and changing style attributes for CSS and reading and adding SVG code to the SVG code in memory (leaving the file code alone).
If it isn't too much work, could you experiment with it a little to see how feasible it is? It would kill two BIG flies in one go if it would work and greatly increase the productivity of Inkscape as an authoring tool. You'd certainly be one of my hero's if you can make this work in Inkscape.
I can also imagine Tav's Symbol tool to increase usability by using external links to symbol files rather than embedding them into the code like it currently does, but that would be food for 0.92 I guess.
Last but not least would be an option to save all linked content into a single whopper of a file and an option to export all style info and <use> objects to external files. Hmm,.. you cannot prevent me from dreaming now can you?
Cheers,
Jelle Mulder