S column fails with tex4ht

[josephwright]

Raised in https://tex.stackexchange.com/questions/707485/how-to-use-make4ht-for-threeparttable-with-siunitx-to-make-an-odt, needs a proper fix.

[josephwright]

I've asked about docs for tex4ht in this area: tracing and trying similar things in collcell, I think the cell collection is OK so it's an issue later in the process. But I need a bit of insight into what is being added, etc., to make progress.