Accordingly to the specs and the valdiator, hyperlinking indivudal pieces of text within a tspan element is absolutely valid.
However, Inkscape currently doesn't allow you to do that. Additionally, FF2 and all Webkit based browsers (e.g. Chrome) won't render those text pieces at all. (Would be ace if someone could go ahead and confirm my Webkit bug report
There will be probably an extension for this a few weeks down the road since I started writing something very similar and it only would need some small changes to cover the functionality you need.