Me to...
You can try to use this hooks: displayAfterCarrier and displayPaymentByBinaries. They display your extra content on delivery methods step (below submit button) and on last step with payment methods (also beloew submit button).
Using JavaScript you can move this code hihger to present it before submit button. I did this.
But... but I can't use, save this values. In PS 1.6 I did this. In PS 1.7 I can't.
I will try this:
what wrote Anthony IVOL.
Because as I see (at screenshots) in this (paid) module fmemodules.com/en/51-custom-fields.html - it's possible in PS 1.7 