I have something using XML::Twig that does this, at least for a subset of the complete CALS spec. It works on tables generated by FrameMaker SGML, so it takes advantages of some of the conventions they use for colspecs, but that could be fixed. More annoyingly I think I remember that it does not process properly row spans (I haven't looked at the code in a while). Doing this would require redesigning the algorithm that places cells on a grid. It also comes with no formal tests, which is the main reason I never released it in the wild.

At least it can give you a starting point.

Let me know if you want it.

I know there is some pretty complete XSLT code that does this, by Norm Walsh I think. even if you don't use it it might also help you.