How to pass long data to a form

I understand how to pass simple data to a form via hidden fields. But what happens when that data is very long? URL querystrings are generally limited to 2048 characters, and my data has the potential to be much longer.

Is there a way to have my form pull that data from an external data source instead of via the querystring? I’m very comfortable developing against an API, so that’s on the table as an option. Or maybe some other option I’m not thinking of?

I’d also rather not have to generate a new form for every person, as that may result in hundreds of forms. 

So my ideal situation: 1 form, pull data from an external source (via a unique identifier I can pass as a hidden variable). 

Thanks!