Dynamically altering second form in a Django wizard

Question

I'm trying to create a Django form wizard so my users can enter model instances into the database in bulk. For this particular model, several of the fields are ManyToMany fields with long lists of related models. Users are almost always adding 10+ instances that share many values for the most tedious-to-enter fields.

So my wizard has two forms: The first a handrolled form that queries the user for common values to be used for all the forms in step two, which is a ModelFormSet. The fields set in the first form should be removed from each form in the second, in order to minimize user terror. So I need to create the ModelFormSet in step two based on the values entered in step one.

I'm using Django 1.7, first of all.

The most obvious solution would be to override the wizard's get_form method and, if we're in step two, use the values from step one to construct a new modelformset:

def get_form(self, step=None, data=None, files=None):
    """In the second step, the WorkSet form needs to be dynamically created."""
    form = super(WorkBulkWizard, self).get_form(step, data, files)
    if step is None:
        step = self.steps.current
    if step == "1":
        d = self.get_cleaned_data_for_step("0")
        fields = ["title"]
        # use values from "d" to add fields to "fields"
        NewFormSet = modelformset_factory(Work, fields=tuple(fields), can_delete=False, extra=10)
        form = NewFormSet(queryset=Work.objects.none())
    return form

This produces the correct formset, but when I submit the form I am returned to a blank version of the same form submission page. No processing, no redirect, and the wizard's done method isn't touched. I have no idea why. If I remove this overloading of the get_form method, and let the wizard display a bog-standard ModelFormSet, then submission works as normal. But that defeats the purpose, as the ModelFormSet is a ghastly nine-screen data-entry extravaganza.

This solution, using Django 1.4, suggests overriding the wizard's get_context_data method to inject my form into the template instead of the form specified in the wizard constructor. That answer uses a Dummy form (an empty forms.Form subclass) as a place holder, and then you replace it much the same way as above, just in a different method:

def get_context_data(self, form, **kwargs):
    context = super(WorkBulkWizard, self).get_context_data(form=form, **kwargs)
    if self.steps.current == "1":
        d = self.get_cleaned_data_for_step("0")
        fields = ["title"]
        # Do the same manipulation of "fields" as above
        NewFormSet = modelformset_factory(Work, fields=tuple(fields), can_delete=False, extra=10)
        form = NewFormSet(queryset=Work.objects.none())
        context = super(WorkBulkWizard, self).get_context_data(form=form, **kwargs)
    return context

But something must have changed since version 1.4, because Django still believes that I've used a Dummy form in the second step, and the data entered in that step is not present in the form_list parameter to the done method.

I'm considering just doing this by hand, using session data, but it seems like the right application for a wizard... Any advice?