I am attempting to create a custom user registration front end form.
I would like to validate some fields and change some fields upon submission /before/ they are saved to the db.

Attempting to use the piklist_save_fields action gets a nasty data structure internal to piklist that has all of the submitted field contents.

I don’t want my code to have to depend on understanding that structure.
Is there a straight-forward way to subscribe to a specific form submission, and get access to modify the $_REQUEST before piklist does its thing?

E.g., I want to set username = md5(email) or maybe just esc_text(email).
I also want to force the role to ” (empty), so I have it as a hidden piklist, but obviously don’t want to allow someone to elevate their role.

The documentation has a list of filters, but not of the actions inside piklist. I found it by grepping.