SMIL seems to be dying these days in favour of css and js-based svg animation. So using some js lib like suggested might be a better choice. But anyway, i need more specific list and figures to say whether i’m willing to work on that.
As for skeleton importing, i have no idea of how bone system currently works. However, i’m slowly progressing with generic reusing feature (github.com/synfig/synfig/issues/272 / Non-destructive waypoint reusing (multiple timelines, etc)) and when i’m done, it shouldn’t be hard to implement skeleton-specific stuff (if it won’t work out of the box). But i think it’s better to discuss bones in a separate thread.