I'm guessing here (I haven't tried it myself), but I don't think this will be possible without a fairly involved customisation exercise. Currently Web Forms support dynamic construction at the time the form is initially requested by the contributor (via dynamic xs:includes), but there's no direct support for forms that are, in part, dynamically built based on content that the user has just entered.
For this scenario you might consider rolling your own form based content entry UI (eg. via a dashlet Web Script) - in this case the XML files generated from the custom dynamic form would still be stored in the user's sandbox etc., it's just that that process wouldn't leverage the default Web Form UI and associated functionality (automatic generation of renditions, for example).
Cheers,
Peter