Since
JSF is a server-side sort of framework, perhaps you can just store the data in a JSF-managed bean, in session scope. Then when the form submits, you can just get the value directly.
This may be unacceptable since your session size may get large. My only other recommendation is to play around with some DHTML and manually hide it. Or, just add it as a hidden input text field in one of the other columns.
Using rendered="false" attribute probably won't work because then the HTMl doesn't get rendered at all, which means the data won't be there. That is, if you do a View Source on the rendered HTML, it won't be there, so when the form submits, you won't see that data.