submit field from another panel?

I have a main form panel with a number of fields and I have another field in another panel that I would like to submit along with the main form panel when that submits.

I could just copy the outlying field value into a hidden field in the main form that's submitting, BUT this is a file upload field. It's in another panel for layout purposes and there is other functionality associated it with it there for image preview sizing and positioning so it's not so easy for me to just move that field into the main form.

So is there any good way to do this is without having to re-work the whole panel and controller structure on the page?

submit field from another panel?