You can repeat any paragraph level content
(ie clone one or more paragraphs or tables).
And I think it has always worked for a repeating run
Certainly you can make a run into a rich text control and add a repeat to that. Then you put a plain text control inside that, and add the content you want. You problem is going to be that your separator (say you put a comma in the rich text control, after the plain text control) appears after the last item as well. Might need to think of an elegant way to handle this. Maybe its just an extra attribute on the repeat tag which specifues the separator text?
You can also repeat a table cell
(following a patch i just committed). I guess you might run out of room horizontally, if your cell width settings are fixed (so don't make them fixed).
As to repeating columns
: not quite at the moment. You can put sectPr in sdtContent (although Word 2007 UI looks a bit strange when you do), but the number of columns is given by w:cols/@w:num, and there is currently no way to change that attribute. It would be interesting to extend the conventions to enable these sorts of non-textual changes.