Pre-populate an inline FormSet?

So, you’re not going to like the answer, partly because I’m not yet done writing the code and partly because it’s a lot of work.

What you need to do, as I discovered when I ran into this myself, is:

1. Spend a lot of time reading through the formset and model-formset code to get a feel for how it all works (not helped by the fact that some of the functionality lives on the formset classes, and some of it lives in factory functions which spit them out). You will need this knowledge in the later steps.
2. Write your own formset class which subclasses from BaseInlineFormSet and accepts initial. The really tricky bit here is that you must override __init__(), and you must make sure that it calls up to BaseFormSet.__init__() rather than using the direct parent or grandparent __init__() (since those are BaseInlineFormSet and BaseModelFormSet, respectively, and neither of them can handle initial data).
3. Write your own subclass of the appropriate admin inline class (in my case it was TabularInline) and override its get_formset method to return the result of inlineformset_factory() using your custom formset class.
4. On the actual ModelAdmin subclass for the model with the inline, override add_view and change_view, and replicate most of the code, but with one big change: build the initial data your formset will need, and pass it to your custom formset (which will be returned by your ModelAdmin‘s get_formsets() method).

I’ve had a few productive chats with Brian and Joseph about improving this for future Django releases; at the moment, the way the model formsets work just make this more trouble than it’s usually worth, but with a bit of API cleanup I think it could be made extremely easy.