Since Caption and TabCaption are display controls on the Portal, I would not recommend using them as a persistence store. They are Variant-typed objects, so their theoretical size is up to 2Gb. But there may be a lower limit because of their roles in the user interface.
What data do you want to persist? How do you want to retrieve it? It may be better to use an INI file or the Windows Registry; the Formativ language supports both alternatives.