Submit signup form from block to page

Problem/Motivation

I have a signup form that appears both as a block and a page. The block appears at the bottom of a very long, complex screen, so if there's an issue with the submission the message is easy for users to miss and is far away from the signup block, whereas the page is much simpler.

I'd really like to have the block be able to submit to the page, so that any error messages, etc. are much easier to see and much more clearly connected to the form. I altered the form to submit to the page, and although I can see the POST data, the submission is ignored by the form on the page.

Steps to reproduce

Alter the block form to submit to the page, submit the block form.

Proposed resolution

Allow the page form to accept submissions from the block form. I'd be happy to take a shot at implementing this, but I can't figure out a path forward. I've tried altering the form IDs on the block form to match the page form: that at least triggered a “The form has become outdated…” message at the top of the page form on submit from the block, but the submission wasn't accepted.

If anyone has any ideas how this might be made to work, I can work on implementation.